A Revision of Snakes of the Genus Conophis Part 6

_Nasal region._--The premaxillary is relatively heavy and has a concavity posteroventrally. The lateral processes slope downward, but remain fairly thick, and do not project far laterally. This shape (fig. 6) tends to strengthen the nasal region; this anterior strengthening may be a reflection of the fossorial habits of these snakes. There are no posterior processes of the premaxillary; thus the line of fusion with the nasals and septo-maxillaries is broad. The nasal plate is more than twice as long as wide. The nasals are relatively flat above, although each curves slightly downward medially and fuses into the medial nasal septum; laterally each nasal is narrower and deflected downward, forming a small dorsal s.h.i.+eld over the nasal cavity. The septo-maxillaries are closely a.s.sociated with the vomers and form the cavity in which the organ of Jacobson is situated. The broad medial part of the septo-maxillary forms the roof and anterior border of the cavity, whereas the anterior part of the vomer contains the main part of the capsule and forms the posterior and most of the lateral borders of the cavity. The vomer has a thin anterior ridge that gradually disappears before it reaches the border of the premaxillary. The vomer is approximately U-shaped, when viewed from below. It has no posterior process and does not articulate with the parasphenoid; there is a sizeable gap between the two bones. The septo-maxillary has a lateral process that terminally is directed slightly anteriorly.

_Cranium and a.s.sociated elements._--The frontal is almost three times as long as it is wide; it is flat above with an emarginate dorsolateral margin that forms the upper limit of the optic capsule.

Ventrally the frontal is concave and forms the median limits of the optic cavity. Farther ventrally the frontal joins with the parasphenoid, which at this place forms the ventral extent of the skull, and together with the basisphenoid forms the ventral part of the posterior three-fourths of the skull. In ventral aspect, the parasphenoid is a long, thin bone, slightly expanded anteriorly. It forms the anterior floor of the optic foramen; whereas the frontal forms the anterior roof of the same opening. The frontal and its septo-maxillary process surround the olfactory fenestra. The prefrontal articulates with the anterolateral process of the frontal.

The posterior surface of the prefrontal forms the anterior wall of the orbit of the eye. The articulating surface upon which the median process of the maxillary bone rests is situated ventrally. The anterior dorsal surface of the prefrontal, together with the anterolateral edge of the frontal, extends slightly over the nasal cavity, affording some degree of protection for the contained organs and forming the posterior border of the cavity. A small nasal process also extends anteriorly from the ventrolateral surface of the prefrontal. The orbital-nasalis foramen is located in the anterior surface of the prefrontal. The parietals are fused into one large bone that forms the roof and sides of the middle part of the cranial cavity. From its suture with the frontal, the dorsal surface of the parietal is relatively flat in the area bounded laterally by the parietal crests, which extend posteromedially from the anterolateral corners of the bone and converge medially at a point near its posterior margin. A slight posterior extension of the parietal crests forms the supratemporal crest, which is present on the posterior part of the parietal and on the anterior part of the supraoccipital. The postfrontals are attached to the anterolateral processes of the parietal. Together the anterior surfaces of these two bones form the posterior rim of the orbit of the eye. The postfrontal extends laterally and ventrally and has a terminal extension that projects anterolaterally. In an articulated skull the trans-palatine articulates with the ventrolateral articulating surface of the postfrontal.

Anteromedially, the parietal forms the roof and posterior margin of the optic foramen. The basisphenoid, which is fused with the parasphenoid, also forms a small part of the posteroventral margin of the optic foramen. The basisphenoid forms the floor of the middle part of the cranial cavity and the ventromedial down-pouching that contains the pituitary body. Posterolateral to the parietal and dorsal to the posterior part of the basisphenoid is the prootic. Laterally this bone is deeply emarginate; posteriorly it forms a large part of the otic notch, through which the columella pa.s.ses. The columella is a long, thin bony rod that terminates posteriorly in cartilage. It is the cartilagenous part of the columella that connects with the external sound detecting mechanism. There are several foramina on the lateral surface of the prootic. On the anterolateral surface of the prootic, branches of the trigeminal nerve pa.s.s through three foramina whereas the facial nerve pa.s.ses through the single posterior foramen near the otic notch. The squamosal is attached dorsoventrally to the posterior part of the parietal and to the lateral part of the prootic.

At this place of attachment there is on the prootic a relatively heavy crest that forms a rather broad articulating base. The squamosal is long, flat, and curves slightly in a dorsal direction throughout its length; it becomes thinner and narrower posteriorly. The posterior third of the squamosal forms a broad base by means of which the squamosal articulates with the quadrate. The columella and the squamosal extend posteriorly beyond the limits of the braincase.

Posteriorly the skull consists of four bones: an unpaired median dorsal supraoccipital, an unpaired median ventral basioccipital and two lateral exoccipitals. The basioccipital does not have noticeable pterygoid processes, but is rather smooth ventrally and only slightly emarginate on its posterolateral margins. Posteriorly, this bone forms the ventral part of the occipital condyle. The rest of the condyle, on each side, is formed by the exoccipitals. The exoccipitals also form part of the base to which the squamosal is attached. The exoccipitals extend around the sides of the foramen magnum and meet dorsally. Each exoccipital also forms the posterior part of the otic notch, which traverses the exoccipital. The exoccipitals bear moderate occipital crests that extend posterolaterally across the supraoccipital as branches from the supratemporal crest. The supraoccipital also has a medial crest that extends a short distance posteriorly from the supratemporal crest onto the exoccipitals at their dorsal line of fusion.

[Ill.u.s.tration: FIG. 7. The maxillo-palatal-pterygoid arch of _Conophis lineatus concolor_ (UMMZ S-788) showing (A) dorsal, (B) lateral, and (C) ventral views. 3. Teeth shown by means of broken lines were represented only by their sockets.]

_Maxillo-palatal-pterygoid arch._--In an articulated skull, the anterior edge of the maxillary is immediately posterior to the lateral tip of the premaxillary (fig. 7). The maxillary is curved moderately laterally and is not robust at its tip, but it becomes heavier about one-third of its length posteriorly. A dorsomedian process begins at about one-third of its distance from the anterior end; the prefrontal articulates with this process. The process is broad and almost flat, except that at its medial end, an elongate, rounded k.n.o.b extends ventrally. The dorsomedian process of the maxillary extends toward, but does not meet, a lateral process from the palatine. The maxillary teeth are set in sockets on the ventral surface of the bone. Just posterior to the level of the last prediastemal tooth is the median trans-palatine process that articulates with the anteromedian part of the trans-palatine. Immediately posterior to this process, the maxillary narrows slightly; then it broadens to form an obliquely oriented k.n.o.b. The posteroventral surface of the posterior k.n.o.b of the maxillary bears one or two enlarged maxillary teeth. (These teeth are discussed further in the section on Dent.i.tion.) The anterolateral part of the trans-palatine articulates with the dorsal surface of the posterior k.n.o.b of the maxillary. Toward the middle of its length, the trans-palatine narrows considerably; then it broadens again and articulates with the pterygoid. The palatine is slightly rounded at its anterior end, which extends anteriorly to the posterior margin of the vacuity containing Jacobson's organ. The palatine extends posteriorly to the trans-palatine process of the maxilla, where the palatine articulates with the pterygoid. A posterior pterygoid process from the palatine projects posteromedially from the end of the palatine and overlaps the anterior end of the pterygoid. Just less than one-half the distance from the anterior end of the palatine, there is a lateral process that curves ventrolaterally forming a blunt tip posteriorly. Slightly more posteriorly and on the medial side of the palatine, is a medial sphenoid process, which is thin, rather broad, and curves ventromedially; ultimately it comes to lie near the anterior part of the parasphenoid. The palatine teeth are set in shallow sockets on the ventral edge of the bone. Of the bones of the maxillo-palatal-pterygoid arch, those on the pterygoid extend farthest posteriorly. The pterygoid is broad medially and posteriorly, although pointed at its posterior tip. The trans-palatine articulates in a broad line at about one-third of the distance along the lateral margin of the pterygoid. Immediately posterior to this articulation, there is a median ridge on the pterygoid; lateral to the pterygoid ridge is an abrupt hollow, the pterygoid groove. Posteromedially, this groove becomes gradually more shallow and disappears. The dorsal surface of the pterygoid is rounded anteriorly and somewhat flattened posteriorly, whereas the ventral surface is gently rounded along its length, except that there is a high median crest. The pterygoid teeth are situated in shallow sockets along this crest. The teeth diminish in size posteriorly.

[Ill.u.s.tration: FIG. 8. The left mandible and a.s.sociated quadrate of _Conophis lineatus concolor_ (UMMZ S-788) showing (A) lateral and (B) medial views. 3. Teeth shown by means of broken lines were represented only by their sockets.]

_Mandible._--The dentary (fig. 8) is compressed laterally and rounded below. The teeth, which are longest about one-third of the way from the anterior end of the dentary, are set in sockets on the medial side of the bone. The posterior half of the dentary overlies the fused surangular-prearticular part of the articular. Ventrally, the posterior part of the dentary underlies the splenial, which is set in a median trench within the dentary. Near the common suture of the dentary and the splenial is the large inferior alveolar foramen; completely within the splenial and ventral to the inferior alveolar foramen is the anterior mylohyoid foramen. Posterior to the splenial and also forming a part of the ventral surface of the mandible is the wedge-shaped angular, which lies directly beneath the fused surangular-prearticular. As has been implied, the articular, the surangular, and the prearticular are fused. The prearticular part of this bone forms a part of Meckel's ca.n.a.l. In the surangular part, immediately posterior to the end of the dentary, is the large surangular foramen. Lying in a longitudinal axis along the medial surface of the articular is a high crest, dorsal to which is a deep hollow. The lateral wall of the articular above this hollow is thin and rounded dorsally; the ventral surface is uniformly round and slightly curved dorsally, except that it ends with a short tympanic crest, which projects beyond the articulation with the quadrate. Where the quadrate articulates with the dorsolateral surface of the posterior portion of the squamosal, the former is broad and has a high mid-lateral crest, which extends about one-third of the distance down the quadrate before disappearing. The columellar process (the place of fusion of the columella) is about two-thirds of the way down the medial surface of the quadrate. Ventrally the quadrate has a narrow neck dorsal to its articulation with the articular. The articulation is formed by two lateral f.l.a.n.g.es of the quadrate that fit over a medial ridge formed by the articular.

Dent.i.tion

Teeth on the maxillary and pterygoid decrease in size posteriorly, whereas those of the dentary do likewise except for the first one or two that are usually slightly smaller than those immediately posterior. The palatine teeth are subequal in size. The enlarged, grooved teeth on the maxillary are in shallow sockets on the posteroventral surface of the posterior k.n.o.b of the maxillary. These teeth point posteriorly. The grooves are deep and are situated anterolaterally. One or two enlarged grooved teeth are present on a given maxillary. There seems to be a correlation between the type of preservation, the age of the snake, and the number of grooved teeth.

Old (large) individuals always have only one grooved tooth that is rooted and functional, whereas some of the younger animals have two in place. Usually replacement teeth are present in alcoholic specimens, but these unrooted teeth are lost in the preparation of dried skeletons. Thus, it seems that in _Conophis_ only one pair of grooved teeth is functional at any one time, although usually replacement teeth are present behind and beside the functional one. Some specimens have one tooth in the medial socket on one side and one in the lateral socket on the other. Replacement teeth on the maxillary and dentary are present in the buccal tissue on the medial side of the bones, whereas on the palatines and pterygoids, the replacement teeth are present laterally. Apparently there are no significant differences in dent.i.tion among the members of the genus _Conophis_.

Vertebrae

The fiftieth vertebra of _Conophis vittatus_ (UMMZ 82642) can be described as follows: The neural spine is elongate, thin and low; the posterior edge is sharply emarginate, and the anterior edge is only slightly emarginate. The zygosphene is thin dorsoventrally; in a ventral or dorsal view the zygosphene has a slightly concave anterior edge, the flat surface of which is oriented ventrolaterally. The centrum is elongate and triangular from below; it is widest at the paradiapophyses and narrowest at the short condylar neck. The condylus is directed posteriorly. The centrum, when viewed laterally, is slightly concave and has prominent subcentral ridges that extend from the median side of the paradiapophysial articular surfaces posteriorly to the neck of the condylus. The paradiapophysial articular surfaces are well developed and have two facets. The diapophysial surface is larger and more spherical than the parapophysial one. The parapophysial process projects beyond the parapophysial articular surface and is nearly even with the lip of the cotyle, which is slightly oval. The neural arch is slightly depressed; its width is somewhat less than the width of the cotyle. The articular surfaces of the postzygapophyses are oval and are directed posterolaterally. There is a strongly developed concave interzygapophysial ridge. A well-developed accessory spine extends laterally beyond the oval articular facets of the pre-zygapophysis and forms a slightly flattened, blunt spine. Excellent drawings of the middle thoracic vertebra of _Conophis lineatus dunni_ from Honduras were published by Auffenberg (1958:6).

Hemipenes

The hemipenes of _Conophis_ are moderately caliculate, having spines covering the surface from the base to near the apex (fig. 9). These spines are largest near the base and are reduced to small papillate projections near the apex. The apex terminates in a small disc having three to five laminae in _C. vittatus_ and one lamina in _C. lineatus concolor_. The sulcus is bifurcate; the fork is near the base and almost gives the appearance of two sulci on some specimens. Distally the apices are widely separated, and the intervening s.p.a.ce gives the hemip.e.n.i.s a slightly bilobed appearance in some species (especially _C. vittatus_) or a deeply bilobed appearance in others (especially _C. lineatus concolor_).

[Ill.u.s.tration: FIG. 9. The everted left hemip.e.n.i.s of _Conophis vittatus_ (UMMZ 82650). 5.]

The everted hemip.e.n.i.s reaches posteriorly to the eighth subcaudal scale. The sulcus bifurcates at the third subcaudal scale. The situation is similar _in situ_ (Cope, 1895:pl. 28, fig. 2).

There are no apparent hemipenial differences among the species of the genus _Conophis_. As can be seen in the above description, the hemip.e.n.i.s of _C. vittatus_ is less bilobed and has a more p.r.o.nounced disc at the apex than the others. The hemip.e.n.i.s of _C. lineatus concolor_ is most bilobed, but has the smallest apical disc. The other species and subspecies vary widely within these extremes.

Food and Feeding

_Conophis_ eats mostly small lizards, especially _Cnemidophorus_. In Mexico _Conophis_ occurs in semi-arid habitat where _Cnemidophorus_ is common. A specimen each of _Conophis vittatus_ and _C. lineatus lineatus_ were obtained while I was collecting _Cnemidophorus_. The only record of _Conophis_ having fed on a warm-blooded vertebrate was obtained in the course of this study, when I recovered from the stomach of a _Conophis lineatus concolor_ (CNHM 36299) from Chichen Itza, Yucatan, a heteromyid rodent (_Heteromys gaumeri_).

Ralph Axtell (personal communication) observed _Conophis_ actively searching for food at dusk. His observations were made near Tehuantepec, Oaxaca, and the snakes were seen to chase lizards of the genus _Cnemidophorus_. Near Alvarado, Veracruz, in the late afternoon, I watched a _Conophis lineatus lineatus_ follow a lizard into a hole.

Mittleman (1944:122) presents the only discussion of the mode of feeding of a captive specimen of _Conophis lineatus_ ssp. When presented with a _Thamnophis_ slightly smaller than itself, the _Conophis_ struck, and within eight minutes immobilized the _Thamnophis_. Within one-half hour the _Thamnophis_ was swallowed.

Three days later the _Conophis_ ate another _Thamnophis_, though still distended from its first meal; nine days later it ate a _Storeria_. In the course of several months, the _Conophis_ ate various toads and hylids and two more _Storeria_. Apparently members of the genus _Conophis_ do not constrict their prey, but rely upon a combination of loss of blood and action of the venom to completely immobilize their prey.

Ditmars (1931:pls. 26-27) showed three photographs of "_Conophis lineatus_" (actually _Conophis pulcher_) ingesting another snake, identified by him as a young _Ophis (= Xenodon) colubrinus_.

Effect of Poison

The rear fangs of these snakes are large for the size of the snake.

Various collectors have been bitten, and several reports of the effect of the poison have been published. The snakes are aggressive and bite constantly while being handled. A field companion, Dale L. Hoyt, was bitten on the forefinger by a specimen of _C. l. lineatus_ and immediately felt a burning sensation. The finger swelled, much as it would if stung by a wasp, but it returned to normal size in about twenty-four hours. Ditmars (1931:legend pl. 27) reported immediate burning pain and a localized swelling, an inch in diameter and half an inch high, which lasted for several hours. Mertens (1952b:83) reported merely that the hand of the gardener at the Inst.i.tuto Tropical in San Salvador bled strongly for a full hour. Edward H. Taylor was bitten by a specimen of _Conophis vittatus_ (Taylor and Smith, 1939:252); pain and swelling lasted for some time. Taylor (personal communication) is still troubled by damage incurred by that bite, which apparently resulted in mechanical damage to the second joint of the middle finger, for the joint swells when the finger is used or exercised.

William E. Duellman (personal communication) was bitten on the hand in July, 1956. There was immediate pain and localized swelling, both of which disappeared several hours later.

TAXONOMIC RELATIONs.h.i.+PS AND EVOLUTION

The genus _Conophis_ is known only from the Recent. Except that _Conophis_ belongs to the subfamily xenodontinae and probably is of New World origin, little is known about the relations.h.i.+ps of the genus. Auffenberg (1958) described a new genus and species of fossil colubrid snake from the Miocene of Montana as _Dryinoides oxyrhachis_ and compared it with several recent genera. This specimen, of which there is a relatively complete skull and a series of vertebrae, seems most closely to resemble a specimen of _Conophis lineatus dunni_ (UF 7657) from Honduras, with which it was compared in basic osteology.

The two genera could be related, for the progenitors of _Conophis_ possibly inhabited much of North America in the Miocene.

Another possibility is that the main stock of the xenodontines reached South America in earliest Tertiary times, and that the formation of the Panamanian and Colombian seaways that separated South America and Central America from the Late Paleocene to the middle of the Pliocene left the _Conophis_ stock isolated in Middle America where members of the genus dispersed through semi-arid habitats.

Turning our attention now to the species within the genus, instead of the genus as a whole, _Conophis vittatus_ is readily set apart from other members of the genus on the basis of the universal presence of seven supral.a.b.i.als. In basic coloration it also differs, having no stripe on the 1st scale-row, or spots on the venter, and a maximum of four broad stripes on the body. The other species appear to be more closely related; these make up the _C. lineatus_-group. _Conophis nevermanni_ differs so much from the other species that it might be placed in a separate group. Nevertheless, the basic striped pattern, which is masked by the increased melanism of many specimens, indicates that _nevermanni_ is more closely related to the _lineatus_-group than to _vittatus_. The _lineatus_-group, thus, consists of _pulcher_, _nevermanni_ and the three subspecies of _lineatus_. In this group the color pattern is characterized by the high frequency of ventral spotting, darkening of part of the supral.a.b.i.als, dark pigmentation on the 1st scale-row, and more than four dark stripes on the body of adults. _Conophis lineatus concolor_, on which the dark pigmentation on the body apparently is secondarily lost, is an exception.

If differences in color pattern be used as an indication of the relations.h.i.+ps between the species and subspecies of the genus _Conophis_, I would consider _C. vittatus_ the most divergent unit.

The subspecies of _lineatus_ closely resemble one another and, as a unit, resemble _pulcher_ from which they differ primarily in the position of the dorsalmost stripes. _Conophis nevermanni_ is more divergent than is _pulcher_ from the species _lineatus_, but probably is not so far removed from _lineatus_ as is _vittatus_.

In the light of what has been pointed out immediately above with respect to resemblances of, and differences between, the species, an hypothesis to account for their formation and for their presence in the areas where they are today is the following: Concurrent with climatic fluctuations in the Late Pliocene and Pleistocene, the northernmost population differentiated into the species _vittatus_, and has subsequently spread north and west from the region of Tehuantepec, Mexico. During the same period _nevermanni_ became isolated in northern Costa Rica.

The species _pulcher_ probably differentiated from the remaining _lineatus_ stock during the Early Pleistocene orogenic upheaval in Guatemala. The _pulcher_ stock was isolated on the Pacific Coastal slopes of Guatemala, while _lineatus_ moved through the subhumid corridor of northern Middle America into Mexico and southward toward Costa Rica (Stuart, 1954a). In the Late Pleistocene and Recent, _pulcher_ moved back across the central Guatemalan highlands occupying its present range in northern Middle America. Primarily because of the formation of unsuitable habitat (wet forest) that presently separates the geographic ranges of populations of _lineatus_, this species differentiated into three subspecies.

SUMMARY

The genus _Conophis_ Peters, 1860, contains four species. Three are monotypic and the fourth has three subspecies, making a total of six taxa.

The genus is characterized by maxillary teeth of equal size followed by a diastema and two enlarged grooved fangs. The scales are smooth, in 19 rows at mid-body, and 17 nearer the tail. The a.n.a.l is divided, apical pits are lacking, the head s.h.i.+elds are normal for a colubrid, and the hemip.e.n.i.s is bilobed having many large basal spines.

The six taxa are separated primarily on the basis of color pattern, but characters of scutellation, including numbers of dorsals, ventrals, caudals, and places of reduction of the number of dorsal scale-rows, were a.n.a.lyzed.

Snakes of this genus are distributed throughout semi-arid environments from southern Mexico southward into Costa Rica. They feed upon lizards, primarily of the genus _Cnemidophorus_; in addition they are known to eat small rodents and other snakes.

_Conophis_ is a member of the subfamily Xenodontinae and, as presently understood, has no known living close relatives. A single specimen of _Dryinoides_ from the Miocene of Montana has been compared with this genus. The genus _Conophis_ is thought to have evolved in Middle America. The present distribution and differentiation probably are primarily the result of climatic fluctuations in Middle America, which produced the areas of subhumid environment where _Conophis_ presently lives.

LITERATURE CITED