The Appendages, Anatomy, and Relationships of Trilobites Part 8

(Text figs. 13-16, 23.)

Ill.u.s.trated: Walcott, Bull. Mus. Comp. Zool., Harvard Coll., vol.

8, 1881, pl. 1, figs. 6-10; pl. 2, figs. 5-7, 10; pl. 3, figs. 1, 3, 8-10; pl. 4, figs. 3, 7; pl. 5, figs. 1-6; pl. 6, figs. 1 (restoration), 2;--Proc. Biol. Soc. Was.h.i.+ngton, vol. 9, 1894, pl.

1. fig. 7 (restoration);--Geol. Mag., dec. 4, vol. 1. 1894, pl. 8, figs. 7, 8;--Smithson. Misc. Coll., vol. 67, 1918, pl. 26, figs.

1-7, 9-13; pl. 27, figs. 4, 5 (not 5a), 11 (not 12, _Ceraurus_), 13, 14, 15 (not _Ceraurus_); pl. 28, figs. 7, 8; pl. 33, fig. 1 (restoration); pl. 34, fig. 2; pl. 35, fig. 6.--Dames, N. Jahrb. f.

Min., etc., vol. 1, 1880, pl. 8, figs. 1-5.--Milne-Edwards, Ann.

Sci. Nat., Zoologie, ser. 6, vol. 12, 1881, pl. 11, figs. 19-32; pl. 12, figs. 33-41.--Packard, Amer. Nat., vol. 16, 1882, p. 796, fig. 12.--Bernard, The Apodidae, 1892, text figs. 50, 52, 54;--Quart. Jour. Geol. Soc., London, vol. 50, 1894, text figs. 13, 15, 17.--Oehlert, Bull. Soc. Geol. France, ser. 3, vol. 24, 1896, fig. 12.--Beecher, Amer. Jour. Sci., vol. 13, 1902, pl. 5, fig. 7.

In both of Walcott's accounts (1881, 1918) of the appendages of _Calymene_ and _Ceraurus_, he has described them together, so that those who have not taken time to study the ill.u.s.trations and disentangle the descriptions are very apt to have a confused notion in regard to them. I have therefore selected from the original specimens those slices of _Calymene_ which are most instructive, and bearing in mind the probable appearance of the appendages of an enrolled _Triarthrus_, have tried to interpret them. In such a method of study, I have of course started with a pre-formed theory of what to expect, but have tried to look for differences as well as likenesses.

_Cephalic Appendages._

_Antennules._--The evidence of antennules rests on a single slice (No.

78). The appendage in question is exceedingly slender and arises at the side of the hypostoma near its posterior end. It shows fine, slender segments, and curves first outward and then forward. If it is in its natural position, it is not an antennule, but the endopodite of the second or third pair of cephalic appendages. It is short, only about one-third the length of the hypostoma, but is doubtless incomplete. The two distal segments show a darker filling, indicating that they were hollow. Judging from a.n.a.logy with other trilobites, the appendage is probably an endopodite and not an antennule. There can be no reasonable doubt, however, that _Calymene_ possessed antennules.

Some idea of the form of the c.o.xopodites of the cephalic appendages may be obtained from sections which cut in approximately the plane of the hypostoma. Such sections are shown in Walcott's photographs (pl.

26, figs. 4, 6, 11, 1918). Specimens 50 (fig. 4, our fig. 13), 51 (fig. 6), 6 (fig. 11), and 40 (our fig. 14) agree in showing two pairs of slender c.o.xopodites which are attached at the sides of the hypostoma and run backward parallel and close to it, and two pairs of larger c.o.xopodites which are behind the hypostoma, although the point of attachment of the third pair is in front of its tip. The anterior pair are apparently under-developed and no longer function as mouth parts, while the posterior two pairs are large and armed on their inner ends with spines. Specimen 78, which has already been mentioned in connection with the antennules, shows a second very slender appendage back of the so-called antennule, which is equally slender, but is directed outward instead of forward. It seems not improbable, from their position and similarity, that these two are the endopodites of the first two appendages on one side of the hypostoma. Specimen 6 shows rather inadequately the endopodites of the second and third cephalic appendages. I have not found other slices showing endopodites of the cephalon. Walcott, in both his restorations, has shown enlarged, paddle-shaped dactylopodites on the distal ends of the fourth cephalic endopodites. The evidence for this rests princ.i.p.ally on three slices, No. 38 (pl. 26, figs. 9, 10), 53 (pl. 26, fig. 12), and 43 (pl. 26, fig. 13). Of these, No. 43 may be dismissed at once as too poorly preserved to be interpreted. No. 53 does show a section of an appendage which seems to have an unusually wide dactylopodite, but this slice presents no evidence at all as to the appendage to which the dactylopodite appertains, nor can one even be sure that there has not been a secondary enlargement. Specimen 43 shows this feature much less definitely than is indicated by the published photograph and drawing. The segment in question is strongly curved, with a constriction possibly dividing it into two. If it is in its natural position in this section, it obviously belongs to one of the thoracic segments and not to the cephalon. With evidence of difference so unsatisfactory, I prefer to reconstruct the posterior cephalic endopodites on the same plan as those of the thorax.

[Ill.u.s.tration: Fig. 13.--Slice through _Calymene senaria_ in the plane of the hypostoma, showing the very slender c.o.xopodites beside that organ, the spines on the inner end of one of the maxillulae, and the anterior position of the attachment of all these appendages. From a photographic enlargement. Specimen 50. 4.]

[Ill.u.s.tration: Fig. 14.--Slice through the hypostoma and thorax of _Calymene senaria_ Conrad, showing the small size of the c.o.xopodites nearest the hypostoma. Sh.e.l.l in black, appendages and filling of abdominal cavity dotted. From a photographic enlargement. Specimen 40.

3.8.]

[Ill.u.s.tration: Fig. 15.--Transverse section of _Calymene_, showing method of articulation with the appendifer. The sh.e.l.l is in solid black, the filling of the appendage and appendifer stippled. Traced from a photographic enlargement of the slice. Specimen 63. 7.]

_Exopodites._--Walcott admits that there is no direct evidence of spiral exopodites in the cephalon of _Calymene_. No one of the sections cutting through the plane of the hypostoma shows any trace of appendages which could be interpreted as exopodites.

_Thoracic Appendages._

The large c.o.xopodites of the anterior thoracic appendages are well shown in many specimens cut longitudinally, of which Nos. 23, 50, and 55 may be mentioned, since photographs of them have been published by Walcott (pl. 26, figs. 1-4, 1918). The endobases of all taper toward the proximal ends. Transverse slices show sections of the c.o.xopodites which are no wider than those in longitudinal sections, indicating that they were not compressed but probably cylindrical. This is borne out by an individual (pl. 28, fig. 7, 1918) which is not a slice but an actual specimen, the body cavity of which was hollow, and, opened from above, shows the impressions of the last two c.o.xopodites of the cephalon, and the first four of the thorax.

One transverse section (No. 63, see our fig. 15) is especially valuable, as it shows the method of articulation of the c.o.xopodites with the dorsal skeleton. Another specimen (No. 73) shows that appendifers are present in _Calymene_, and while the appendifer does not retain its original form in slice No. 63, the section does show clearly that there was a notch in the inner (upper) side of the c.o.xopodite into which the lower end of the appendifer fitted, thus giving a firm, articulated support for the appendage. This notch appears to be slightly nearer the outer than the inner end of the c.o.xopodite, and since it must have made a kind of ball-and-socket joint, considerable freedom of movement was allowed. The appendage must have been held in place by muscles within the c.o.xopodite and attached to the appendifer.

No slice which I have seen shows a continuous section through all the segments of an endopodite, but many, both longitudinal and transverse, show one, two, or as many as three segments.

Such sections as No. 120 show that the endopodites of the thorax were slender and composed of segments of rather uniform diameter.

Other sections, notably No. 83, 154, and in, show that they tapered distally, and bore small spines at the outer end of each segment.

The exopodites of course furnish the chief difficulty in interpretation. Doctor Walcott finds two sets of structures attached to the c.o.xopodite, a long, slender, spiral exopodite, and a short, broad epipodite with a fringe of long setae. Since he has given the same interpretation for _Calymene_, _Ceraurus_, and _Acidaspis_, I have considered the question of all three together on a preceding page (p. 48), and given my reasons for regarding both structures as due to sections in different directions across setiferous exopodites.

Sections like those shown in figures 11, 13, and 14 of plate 27 (1918) happen to be cut in or near the plane of the setae of an exopodite, and so show hairs of considerable length. Such sections are, as would be expected, very few in number, while sections like those shown on figures 4, 5, 7, and 9 of plate 27, which cut the setae more nearly at right angles, are very common. Slices which give any definite idea of the form of the shaft of the exopodite are exceedingly rare. Perhaps the most satisfactory one is No. 23 (pl. 3, fig. 3, 1881), which shows the proximal part of a long, slender, unsegmented shaft, with the bases of a number of slender setae. The organ is not complete, as would be inferred from the published figure, but the section cuts diagonally across it, and the total length is unknown. It is directed forward, like the exopodites of Neolenus, but whether or not this is a natural position is yet to be learned.

The proximal, non-setiferous portion of the exopodite is evidently at an angle with the setiferous part. Another similar exopodite is apparently shown by specimen 29 (pl. 3, fig. 9, 1881), which has a similar angulated shaft and just a trace of the bases of the setae.

_Pygidial Appendages._

That appendages were present under the pygidium is shown by longitudinal sections, but nothing is known of the detail of structure.

[Ill.u.s.tration: Fig. 16. Restoration of _Calymene senaria_ Conrad, based upon data obtained from the study of the translucent sections made by Doctor Walcott. Prepared by Doctor Elvira Wood, under the supervision of the author. About twice natural size.]

_Relation of Hypostoma to Cephalon in Calymene._

In _Calymene_ the shape of the hypostoma bears little relation to the shape of the glabella, and it is relatively smaller, both shorter and narrower, than in Ceraurus. In shape, neglecting the side lappets at the front, it is somewhat rectangular, but rounded at the back, where it is bifurcated by a shallow notch. The anterior edge has a narrow f.l.a.n.g.e all across, which is turned at almost right angles to the plane of the appendage, and which fits against the doublure of the free cheeks at the sides and against the epistoma in the middle. The side lappets show on their inner (upper) surface shallow pits, one on each lappet, which fit over projections that on the dorsal surface show as deep pits in the bottom of the dorsal furrows in front of the anterior glabellar furrows. The appendifers on the head in _Calymene_ take the form of curving projections of sh.e.l.l underneath the glabellar and neck furrows, and owing to the narrowness of the hypostoma, all these are visible from the ventral side, even with it in position. This s.h.i.+eld extends back about 0.6 of the length of the cephalon, and to a point a little behind the second glabellar furrow from the back of the head.

In Doctor Walcott's restoration of _Calymene_ he has represented all four pairs of biramous appendages as articulating back of the posterior end of the hypostoma. I think his sections indicate that the gnathobases of two pairs of these appendages rested alongside or beneath it, and in particular, the longitudinal sections (1881, pl. 5) would appear to show that the mouth was some distance in advance of its posterior end.

_Restoration of Calymene._

(Text fig. 16.)

From what has been said above, it is evident that for a restoration of the appendages of _Calymene_ considerable dependence must be placed upon a.n.a.logy with other trilobites. Nothing is positively known of the antennules, the exopodites of the cephalon, or any appendages, other than c.o.xopodites, of the pygidium, but all were probably present. It is inferred from the slices that the first two pairs of cephalic appendages were poorly developed, the endopodites short and very slender, the c.o.xopodites lying parallel to the sides of the hypostoma and nearly or quite functionless. The gnathites of the last two pairs of cephalic appendages are large, closely approximated at their inner ends, and bear small tooth-like spines. The endopodites are probably somewhat better developed than the anterior ones and more like those on the thorax.

The c.o.xopodites of the thorax appear to have had nearly cylindrical endobases which tapered inward. The endopodites were slender, tapering gradually outward, and probably did not extend beyond the dorsal test.

Small spines were present on the distal end of each segment. Each exopodite had a long, slender, unsegmented shaft, to which were attached numerous long, overlapping, flattened setae. The shaft may have been angulated near the proximal end, and may have been directed somewhat forward and outward as in Neolenus, but the evidence on this point is unsatisfactory. The number of pairs of appendages is that determined by Walcott from longitudinal sections, namely, four pairs on the cephalon beside the antennules, thirteen pairs in the thorax, and nine pairs on the pygidium.

=Calymene= sp. ind.

(pl. 6, figs. 4, 5.)

Ill.u.s.trated: Walcott, Bull. Mus. Comp. Zool., Harvard Coll., vol.

8, 1881, pl. 6, figs. 5a, b;--Proc. Biol. Soc. Was.h.i.+ngton, vol. 9, 1894, pl. 1, fig. 10;--Geol. Mag., dec. 4, vol. 1, 1894, pl. 8, fig. 10;--Smithson. Misc. Coll., vol. 67, 1918, pl. 36, figs. 1, 2, 2a-d.--Milne-Edwards, Ann. Sci. Nat., Zoologie, ser. 6, vol. 12, 1881; pl. 12, figs. 44a, b.

In the United States National Museum there is a thin piece of limestone, about 3 inches square, which has on its surface eight jointed objects that have been called legs of trilobites. Two of these were figured by Walcott (1881, pl. 6, fig. 5). The slab contains specimens of _Dalmanella_ and _Cryptolithus_, in addition to the appendages of trilobites, and is said by Doctor Ulrich to have come from the tipper part of the Point Pleasant formation (Trenton) on the bank of the Ohio River below Covington, Kentucky.

The specimens are all endopodites of long slender form, similar to those of _Triarthrus_, but since that genus does not occur in the Point Pleasant, it is necessary to look upon some other trilobite as the former possessor of these organs. Both _Isotelus_ and _Calymene_ occur at this horizon, and as the specimens obviously do not belong to _Isotelus_ or _Cryptolithus_, it is probable that they were formerly part of a _Calymene_.

All the endopodites are of chitinous material, and the various specimens show, according to the perfection of their preservation, from four to six segments. The endopodite as a whole tapers but slightly outward, and the individual segments are of nearly equal length. They appear to be but little crushed, and are oval in section, with a crimped anterior and posterior margin. One or two show a median longitudinal ridge, such as is seen in some appendages of _Triarthrus_. Each segment is parallel-sided, with a slight expansion at the distal end, where the next segment fits into it.

Under the heading "Ordovician Crustacean Leg," Walcott (1918, p. 154, pl. 36, figs. 1,2) has recently redescribed these specimens, and thinks that they do not belong to _Calymene_, nor, indeed, to any trilobite. He concludes that they were more like what one would expect in an isopod. Pa.s.sing over the fact that the oldest isopod now known is Devonian, the fossils in question seem to me quite trilobite-like.

Walcott says:

The legs are a.s.sociated with fragments of _Calymene meeki_ but it is not probable that they belong to that species; if they did, they are unlike any trilobite leg known to me. The very short c.o.xopodite and basopodite are unknown in the trilobites of which we have the legs, as they are fused into one joint forming the long protopodite in the trilobite. The distal joint is also unlike that of the trilobite legs known to us.

A great deal of Doctor Walcott's difficulty probably arises from his h.o.m.ology of the c.o.xopodite of the trilobite with the protopodite of the higher Crustacea. The c.o.xopodite of the trilobite is not fused with the basipodite, this latter segment always remaining free.

Indeed, Walcott himself says of _Neolenus_ (1918, p. 128):

Each thoracic leg (endopodite) is formed of a large elongate proximal joint (protopodite), four strong joints each about 1.5 times as long as wide (basopodite, ischiopodite, meropodite and carpopodite); two slender elongate joints (propodite and dactylopodite) and a claw-like, more or less tripart.i.te termination.

Walcott's drawing (pl. 36, fig. 1) is a composite one, and while it shows eight segments, I was not able to count more than seven on any of the specimens themselves. In regard to the terminal segment, the dactylopodite of the limb shown in his plate 36, figure 2, is unusually long, and a comparison with other photographs published on the same plate shows that such long segments are unusual.

Proof that these are appendages of a _Calymene_ is of course wanting, but there is no particular reason so far to say that they are not.

_Measurements:_ Two of the more complete specimens, each showing six segments, are each 8 mm. long.

Somewhat similar to the specimens from Covington are the ones described by Eichwald (1825, p. 39, 1860, pl. 21), the specimens being from the Silurian of Gotland. The figure copied by Walcott (1881, pl.

6, fig. 4) has never been looked upon as entirely satisfactory evidence of the nature of the specimen, and so far as I know, the fossil has not been seen by any modern investigator.