The Cambridge Natural History Part 42

This species differs from its ally by the fact that it often turns white in winter with the exception of the black tips to the ears. In Ireland this change does not always occur; but Mr. Barrett-Hamilton has commented upon the fact that Hares of this species do change on Irish mountains. It appears that in this animal the change from the winter to the summer dress is accomplished by the actual casting off of the white hairs and their replacement by a fresh growth of "blue" hairs. A similar change occurs in the American _L. america.n.u.s_.

Dr. Forsyth Major has noted the fact that the various species of Hares can be distinguished by the condition of the furrows upon the upper incisors.

Thus two African species, _L. crawshayi_ and _L. whytei_, are to be separated by the fact that in the former the incisors are quite flat, whereas in _L. whytei_ the groove is more prominent and there is a second shallow furrow.

The genus _Romerolagus_[377] is quite a recent discovery. It occurs on the slopes of Popocatepetl in Mexico; it has the general aspect of the last genus, and is spoken of as a "Rabbit." It inhabits runs in the long gra.s.s which clothes the sides of the {505} mountain. Externally it is something like the Pikas, since it has no tail visible. The ears, too, are short, and the hind-legs comparatively short. The skull is very like that of the Rabbit; but in other osteological details it is aberrant. Thus the clavicle is quite complete, and only six ribs articulate with the sternum, instead of the seven that we find in the Rabbit.

FAM. 2. LAGOMYIDAE.--The animals of this family are smaller than the Hares and Rabbits; they have short Vole-like ears and no external tail. The limbs also appear to be shorter. As there is but a single genus, the characters of the family may be described in connexion with those of the genus, which is known as _Lagomys_ (apparently more correctly _Ochotona_). Of this genus there are about sixteen species, which are mainly Asiatic; one species extends its range into Eastern Europe, and three are North American.

The skull has not the supra-orbital grooves of the Rabbits, and has a well-marked backward process of the zygomatic arch. There are eighteen dorsal vertebrae. The molars and premolars are five.

The vernacular names of "Pika" and "Piping Hares" have been applied to the members of this genus, the latter on account of their peculiar call. They live among rocks in companies and they burrow. They are usually found at considerable alt.i.tudes: thus _L. roylei_, the "Himalayan Mouse Hare," is found at elevations as high as 16,000 feet; while _L. ladacensis_ gets even higher, 19,000 feet having been recorded. With the habits of a Marmot, so far as concerns living in burrows and at great alt.i.tudes, the animals of this genus, with their squat form and short ears, are not unlike those animals. In the past this genus occurred more generally over Europe.

Species from Miocene beds have been met with in England, France, Germany, and Italy.

FOSSIL RODENTS.--Quite a large number of existing genera of Rodents are known from even the earlier strata of the Tertiary period. The Squirrels (and even the genus _Sciurus_ itself) occur in the Upper Eocene. So, too, do the genera _Myoxus_, and (in South America) _Lagostomus_.

_Spermophilus_, _Acomys_, _Hystrix_, _Lagomys_, _Lepus_, _Hesperomys_ are known from Miocene rocks. _Rhizomys_, _Castor_, _Cricetus_, _Mus_, _Microtus_, and some others appear to have originated so far as we know in the Pliocene, while a still larger series of existing genera are Pleistocene. It is interesting {506} to note that some of the extinct genera were much larger than recent forms. At present, _Hydroch.o.e.rus_ is the biggest Rodent; but the genus _Megamys_ from the Pampas formation of Argentina was "nearly as large as an ox." The wider range of genera in the past is ill.u.s.trated by _Hystrix_, which, now an Old-World form, is represented by remains in the Miocene and Pliocene of America.

It is a significant fact that of living genera _Sciurus_ is the oldest; for it has been pointed out that in a number of features the Squirrels are among the most primitive of Rodents. The zygomatic arch is slender, and has thus not acquired the specialisation that is to be found in that part of the skull in other Rodents; moreover, the "jugal bone is not supported by any process from the maxilla exactly as in the primitive Ungulata." The feet, too, are unspecialised, though that is the case with many other genera. It may also be pointed out that the teeth bear not a little likeness to those of _Ornithorhynchus_ in their mult.i.tuberculate character.

Some few fossil forms have already been dealt with in the preceding pages.

The two genera _Castoroides_ and _Amblyrhiza_, from the Pleistocene of North America and the West Indies, are usually regarded as forming a family. The skull of the former genus indicates an animal of the size of a Bear. It is compared to that of _Castor_, but it has a wide infra-orbital foramen. The teeth are four in each jaw, and are formed of three to five lamellae; the incisors of this animal are powerful but short. _Amblyrhiza_, on the other hand, has long incisors which are longitudinally grooved anteriorly. It has a free fibula. This latter as well as other characters have led Tullberg to remove it from a.s.sociation with _Castoroides_.

ORDER X. TILLODONTIA.

This group of Eocene mammals is to be defined by a number of characters, of which the more important are the following:--The incisors are enlarged, grow from persistent pulps, and are coated with enamel upon the outer surface only; they are those of the second pair only, the first and third having disappeared or become small. The canines are reduced in the later forms. {507}

[Ill.u.s.tration]

FIG. 247.--_Tillotherium fodiens._ Left lateral view of skull. (From Flower, after Marsh.)

These animals have been regarded as ancestral Rodents, to which the tooth characters just mentioned clearly show likenesses. The earliest known form is _Esthonyx_. This genus shows such primitive characters, compared with its later representatives, as the existence of all three pairs of incisors in the upper jaw, but only two in the lower jaw. The enlarged incisors of both jaws do not seem to have grown from persistent pulps.

_Anchippodus_, a later form, still preserves the upper pair of first incisors in a vestigial form; the strong second incisors grew from persistent pulps. The most recent genus, _Tillotherium_, shows the characteristics of the group at their height. The strong Rodent-like, chisel-shaped incisors, which are reinforced by a small additional pair in the upper jaws only, are persistent. The grinding teeth are of the tritubercular pattern; there are three of each kind in the upper jaw, but in the lower jaw only two premolars on each side. This is at any rate the case with some, while others have three. The canine, though present in both jaws, is insignificant. As in many ancient types, there is an entepicondylar foramen in the humerus. The feet were five-toed, and bore sharp, laterally-compressed claws. The skull has been compared in general aspect to that of a Bear.

{508}

CHAPTER XVI

INSECTIVORA--CHIROPTERA

ORDER XI. INSECTIVORA.

The Insectivora[378] are an order of mammals to which it is (to quote Professor Huxley) "exceedingly difficult to give a definition." They are, however, none of them large animals, and most of them are nocturnal in habit--two circ.u.mstances which may have had something to do with their survival from past ages, as may have also their modification to so many and diverse modes of life; for everything points to the antiquity of the group.

They are, for instance, more or less plantigrade. The snout is generally long, and is often prolonged into a short proboscis.[379] There is a tendency for the teeth to be of a generalised type, and their number is often the typical mammalian forty-four. Moreover, trituberculate teeth, which are certainly an ancient form of tooth, are common; and indeed the Insectivora of the southern regions of the globe, _e.g._ Centetidae, Solenodontidae, and Chrysochloridae, have the most prevalent trituberculism, a fact which is of importance in considering the age of the animal life of these regions of the world. The limbs are, as a rule, provided with five digits. The hemispheres of the brain are usually smooth, and do not extend over the cerebellum. The palate is often fenestrate as in the Marsupials, and as in that group the lower jaw is sometimes inflected.

But the latter character also occurs in the Sea-lions and elsewhere.

Clavicles are present, as a rule, but not in _Potamogale_. {509}

There is, furthermore, a distinct tendency towards a disappearance of functional milk teeth, which is best seen in _Sorex_, where there are only seven milk teeth, none of which ever cut the gum. This suppression of the milk dent.i.tion is like that of the Marsupials, Edentates, and Whales, all of which appear to be--the first certainly are--ancient forms of mammalian life.

There is also a fairly well-defined, though shallow, cloaca in many genera.

Finally, the testes are purely abdominal in some, and in none is there a full descent into a s.c.r.o.t.u.m, as in the more highly-developed Eutheria.

SUB-ORDER 1. INSECTIVORA VERA.

FAM. 1. ERINACEIDAE.--This family contains the genera _Erinaceus_, _Hylomys_, and _Gymnura_.

_Hylomys_, considered by Dobson to fall within _Gymnura_, is kept separate by Leche.[380] _H. suillus_ is a Malayan animal, small in size, about 5 inches long, with a short tail. Like _Gymnura_ it is spineless. The ears are decidedly large and nude. There is one pair of inguinal and one pair of thoracic teats. The colour above is a rusty brown with yellowish-white under parts. The palms and soles are quite naked. In its general form it recalls _Tupaia_ very much more than its own immediate relatives. There is no doubt, however, of its systematic position when the skeleton and teeth are examined. A variety has been described from alt.i.tudes of 3000 to 8000 feet on Mount Kina Balu in Borneo. It has the complete dent.i.tion of forty-four teeth. There are fourteen pairs of ribs. As in _Gymnura_ the tibia and fibula are united below. The genus is considered by Leche to be the oldest existing type of Erinaceidae.

_Gymnura_[381] is also a Malayan form with the complete dent.i.tion of the last, but with fifteen pairs of ribs and a longer tail, consisting of twenty-three vertebrae as against fourteen. There is, as with _Hylomys_, but one species, _G. rafflesii_. This animal has a peculiar odour, resembling decomposed cooked vegetables. {510} The under surface of the tail is rough, and it is thought by Dr. Blanford that it may be of use to the animal in climbing. Its compressed terminal third and the fringe of stiff bristles on the under surface of this indicate, according to Dr.

Dobson, powers of swimming, or at any rate a not very remote ancestry of swimming creatures. It is purely insectivorous in diet.

_Erinaceus_, including the Hedgehogs, is a widely distributed genus--Palaearctic, Oriental, and Ethiopian in range. There are about twenty species. The familiar spines distinguish the Hedgehogs from their allies, as also the fact that they possess but thirty-six teeth, the formula being I 3/2 C 1/1 Pm 3/2 M 3/3. There are fifteen or fourteen ribs, and the tail is very short, consisting of only twelve vertebrae. As in _Gymnura_ there is no caec.u.m. The upper canine has usually, as in other Erinaceidae, two roots, but not in _E. europaeus_, which is one of the most modified of Hedgehogs.

The Hedgehog is a more omnivorous creature than _Gymnura_. It eats not only insects and slugs, but also chickens and young game birds, and lastly vipers. Four, or in some cases as many as five or six, young are produced at a birth; they are blind, with soft and flexible white spines. In hot and dry weather Hedgehogs disappear; they come forth in rainy weather. The English Hedgehog, as is well known, hibernates. The Indian species do not.

The Hedgehog is occasionally spineless, which condition may be regarded as an atavistic reversion.[382]

The Hedgehog has acquired the reputation of carrying off apples transfixed upon its spines. Blumenbach thus quaintly describes this and other habits of the animal, whose English name he gives as "hedgidog": "Il se nourrit des productions des deux regnes organises, miaule comme un chat, et peut avaler une quant.i.te enorme de mouches cantharides. Il est certain qu'il pique les fruits avec les epines de son dos, et les porte ainsi dans son terrier."[383]

The Miocene _Palaeoerinaceus_ is so little different from _Erinaceus_ that it is really hardly generically separable. _Erinaceus_ is therefore clearly one of the oldest living genera of mammals.

_Necrogymnura_ of the same epoch and the same beds (Quercy Phosphorites) is doubtless an ancestral form. The palate is {511} perforated as in _Erinaceus_ (it is not so in _Gymnura_ and _Hylomys_), but on the whole it comes nearest to _Hylomys_.

FAM. 2. TUPAIIDAE.--This family contains the genera _Tupaia_ and _Ptilocercus_. _Tupaia_ is Oriental in range, extending as far east as Borneo. There are a dozen or so of species, which are generally arboreal and have the outward aspect of Squirrels. It has been suggested that this is a case of mimicry, the animal gaining some advantage by its likeness to the Rodent. The name Tupaia, it should be added, means Squirrel, and the long-nosed Squirrel, _Sciurus laticaudatus_, is so extraordinarily like it that "one has to look at the teeth" to distinguish them. Moreover this Squirrel, like some Tupaias, lives largely on the ground among fallen logs.

_Tupaia_ resembles a Lemur in the complete orbit. The dental formula is I 2/3 C 1/1 Pm 3/3 M 3/3 = 38. The sublingua, too, is stated by Garrod to be like that of _Chiromys_. There is a minute caec.u.m in _T. belangeri_, none in _T. tana_.

_Ptilocercus_[384] has a pen-like posterior portion to the tail, a modification which is found in other groups of animals. The tail of certain Phalangers, for instance, shows this same modification. The rest of the tail is scaly. The animal, as was pointed out by Dr. Gray,[385] looks very much like a Phalanger. The orbit is entire as in _Tupaia_. The fingers and toes are five. The one species, called after Sir Hugh Low, G.C.M.G., _P.

lowi_, is a Bornean animal.

FAM. 3. CENTETIDAE.--This family is entirely confined to the Island of Madagascar. It includes some seven genera. The best-known genus is _Centetes_. _C. ecaudatus_, the Tanrec, Tenrec, or Tendrac, is an animal a foot or so in length, without a tail, and with forty-four teeth.[386] The immature animal is so different from the parent as to appear quite a different form. It has three narrow rows of spines along the back, which do not wholly disappear until the permanent dent.i.tion has been acquired. Even then the hairs are of a rather spiny character, particularly those upon the back of the head, which are erected when the animal is {512} annoyed. The Tanrec feeds mainly upon earthworms. It is "probably the most prolific of all animals," since as many as twenty-one young are said to have been brought forth at a birth. Some Opossums, however, have twenty-five teats.

[Ill.u.s.tration]

FIG. 248.--Skull of Tenrec. _Centetes ecaudatus._ _fr_, Frontal; _max_, maxilla; _pa_, parietal; _p.max_, premaxilla; _sq_, squamosal. (After Dobson.)

_Hemicentetes_[387] is a genus with two species. These have spines mixed with the fur of the back. There is no caec.u.m in this or in other Centetidae. The teeth are forty in number, there being only three molars.

_Ericulus setosus_ is a small Insectivore, resembling externally a small Hedgehog. It is covered with close-set spines which, unlike what is found in _Erinaceus_, extend over the short tail. The total number of teeth is thirty-six, the formula being I 2/2 C 1/1 Pm 3/3 M 3/3.

_Echinops_[388] is another spiny genus which is a stage in advance of _Ericulus_, for still another molar has been lost, reducing the total number of teeth to thirty-two. The dental formula is thus I 2/2 C 1/1 Pm 3/3 M 2/2. The zygomata are reduced to mere threads.

_Microgale_, a genus recently inst.i.tuted by Mr. Thomas, is a small furry Insectivore with a long tail, which is more than double the length of the head and body. There are no less than forty-seven vertebrae in the tail, which is relatively longer than that of any other mammal.

_Limnogale_, discovered by Forsyth Major, is an aquatic genus, also furry and not spiny, which has departed from the Centetid type and taken to an aquatic life. The single species, {513} _L. mergulus_, is about the size of _Mus rattus_; it has webbed toes and a powerful laterally-compressed tail.

Clavicles are present, which is not the case with _Potamogale_.

_Oryzoryctes_ is a Mole-like Centetid. It has fossorial fore-limbs, but a fairly long tail. This genus is furry like the last two. It is said to burrow in the rice-fields and to do much harm. The teeth are forty in number, three incisors and three molars in each half of each jaw.