Part 50 (1/2)
SIZE.--From 5 to 7 feet long usually, but said to reach 10.
Dr. Kellaart says that at an early age this animal has as many as 32 teeth, viz. inc. 4/8, and molars 5--5/5--5, but when adult there are only 14, as mentioned above. The molars, according to Dr. Murie, succeed each other, the fore ones dropping out, and others from behind taking their places. It feeds on fucus and other seaweeds, and the flesh is considered good eating, and not unlike veal or, some say, pork. They are lethargic in disposition, and in those countries where they have been unmolested they are so fearless of man as to allow themselves to be handled--a confidence somewhat betrayed by the natives, who on such occasions manage to abstract the fattest calves, which are considered a delicacy.
ORDER RODENTIA.
THE GNAWERS.
This order, GLIRES of Linnaeus and his followers, is composed of animals, chiefly of small size, which differ from all others by the peculiarity of their teeth. No one, even though he be most ignorant of comparative anatomy, could mistake the rat or rabbit-like skull of a rodent for that of any other creature. The peculiar pincer-like form of the jaws, with their curved chisel-shaped teeth in front, mark the order at a glance. There is no complexity in their dent.i.tion.
There are the cutters or incisors, and the grinders; and of the cutters there are never more than two in each jaw, that is to say efficient and visible teeth, for there are in some species rudimentary incisors, especially in the young, but these either disappear or take no part in work. Between the grinders and incisors are toothless gaps. The formation and growth of the teeth are peculiar; and it is strange that the gigantic elephant should be the nearest approach to these small creatures in this respect. The teeth--in most cases the grinders, but always the incisors--grow continuously from a persistent pulp, and therefore loss from attrition is kept constantly supplied by growth from behind. The incisors are planted in a socket which is the segment of a circle.
These segments are not equal in both jaws. The lower one is a small segment of a large circle, the upper one is the reverse, being a larger segment of a smaller circle. The angle at which they meet is always the same. Some curious malformations are occasionally found which ill.u.s.trate the growth of these teeth. Should by any chance, accident or design, one of these incisors get diverted from its proper angle and not meet with the friction which is necessary to keep it in its normal condition, it goes on growing and growing, following its natural curve till it forms a ring, or by penetrating the mouth interferes with the animal's feeding. A case is recorded by Blyth of a rat which had an eye destroyed by a tooth growing into it. Here again occurs a similarity to the elephant, whose tusks grow in the same manner, and if abnormally deflected will occasion, as in the case of one lately described to me, serious hindrance to the movement of the trunk. The incisors of rodents are composed of dentine coated in front with a layer of hard enamel, the other surfaces being without this protection, except in the case of some, amongst which are the hares and rabbits, which have a thin coating as well all over. These forms are those with rudimentary incisors, and const.i.tute the links connecting the other mammalia with the Gnawers.
The molars are much alike in structure, and can hardly be divided, as they are by some naturalists, into molars and premolars. They take the three hindmost as molars, regarding the others as premolars.
Sometimes these grinders have roots, but are more commonly open at the end and grow from a permanent pulp. They are composed of tubular and convoluted portions of enamel filed up with dentine, and their worn surfaces show a variety of patterns, as in the case of the Proboscidea. These enamelled eminences are always transverse, and according to Cuvier those genera in which these eminences are simple lines, and the crown is very flat, are more exclusively frugivorous; others, in which the teeth are divided into blunt tubercles, are omnivorous; whilst some few, which have no points, more readily attack either animals, and approximate somewhat to the Carnivora.
The head is small in proportion to the body, the skull being long and flat above; the nasal bones are elongated; the premaxillaries very large on account of the size of the incisor teeth, and the maxillaries are, therefore, pushed back; the zygomatic arch is well developed in most, but is in general weak; the orbit of the eye is never closed behind; the tympanic bulla is very large; the jaw is articulated in a singular manner; instead of the lateral and semi-rotary action of the Herbivora, or the vertical cutting one of the flesh-eating mammals, the rodent has a longitudinal motion given by the arrangement of the lower jaw, the condyle of which is not transverse, but parallel with the median line of the skull, and the glenoid fossa, or cavity into which it fits, and which is situated on the under side of the posterior root of the zygoma, is so open in front as to allow of a backwards and forwards sliding action. The vertebral column is remarkable for the great transverse processes directed downwards, forwards, and widening at the ends. In the hare these processes are largely developed; the metapophyses or larger projections on each side of the central spinous process are very long, projecting upwards and forwards; the anapophyses or smaller projection in rear of the above are small; and the hypapophyses or downward processes are remarkably long, single and compressed; according to Professor Flower these latter are not found in the Rodentia generally. The tail varies greatly, being in some very small indeed, whilst in others it exceeds the length of the body; the sternum or breast-bone is narrow and long, and collar-bones are to be found in most of the genera; the pelvis is long and narrow. In most cases the hind limbs are longer and more powerful than the fore-limbs; in some, as in the jerboas (_Dipus_) and the Cape jumping hare (_Pedetes caffer_), attaining as disproportionate a length as in the kangaroos, their mode of progression being the same; the tibia and fibula are anchylosed; the forelimbs in the majority of this order are short, and are used as hands in holding the food to the mouth, the radius and ulna being distinct, and capable of rotatory motion. The feet have usually five toes, but in some the hind feet have only four, and even three. In point of intelligence, the rodents do not come up to other mammals, being as a rule timid and stupid; the brain is small and remarkably free from convolution. The cerebellum is distinctly separated from and not overlapped by the hemispheres of the cerebrum; the organs of smell, sight and hearing are usually well developed; the stomach is simple or in two sacs; the intestinal ca.n.a.l and caec.u.m long. The latter is wanting in one family.
Rodents have been divided in various ways by different authors.
Jerdon separates his into four groups, viz. ”_Sciuridae_, squirrels; _Muridae_, rats; _Hystricidae_, porcupines; and _Leporidae_, hares; which indeed are considered by some to embrace the whole of the order; to which has recently been added the _Saccomyidae_, or pouched rats, whilst many systematists make separate families of the dormice, _Myoxidae_; jerboas, Dipodidae; voles, _Arvidolidae_; mole-rats, _Aspalacidae_ and _Bathyergidae_; all included in the MURIDAE; and the _Caviadae_, _Octodontidae_, and _Hydroch.o.e.ridae_, belonging to the HYSTRICIDAE” ('Mammals of India,' p. 164).
However, the system that most commends itself is that of Mr.
E. R. Alston, proposed in the 'Proceedings' of the Zoological Society, and founded on the original scheme of Professor Gervais, by which the order is subdivided into two on the character of the incisor teeth.
Those which have never more than two incisors, coated only in front with enamel are termed SIMPLICIDENTATA, or _Simple-toothed Rodents_.
The other sub-order, the genera of which have rudimentary incisors, as in the case of hares, rabbits, &c., and in which the enamel is spread more or less over all the surface, is termed DUPLICIDENTATA or _Double-toothed Rodents_, and this is the system I propose to follow.
SUB-ORDER SIMPLICIDENTATA.
SIMPLE-TOOTHED RODENTS.
These, as I before observed, are those of the order which never have more than two incisors in the upper jaw, and the enamel on these is restricted to the front of the tooth. They have also a well-developed bony palate, which in the Duplicidentata is imperfect, forming in fact but a narrow bridge from one jaw to the other. In the latter also the fibula, which is anchylosed to the end of the tibia, articulates with the calcaneum or heel-bone, which is not the case with the simple-toothed rodents.
We now come to the subdivisions of the Simplicidentata. The order GLIRES has always been a puzzling one to naturalists, from the immense variety of forms, with their intricate affinities, and there is not much help to be gained from extinct forms, for such as have been found are mostly referable to existing families. The cla.s.sification which I have adopted is, as I said before, that elaborated by Mr. E. R. Alston, F.G.S., F.Z.S., and reported in the 'Proceedings' of the Zoological Society for 1876. I said that he had founded it on Professor Gervais' scheme, but I see that the groundwork of the system was laid down in 1839 by Mr. G. R. Waterhouse, then curator of the Zoological Society, and it was afterwards, in 1848, taken up by Professor Gervais, and subsequently added to by Professor Brandt in 1855, and Lilljeborg in 1866. About ten years later Mr. Alston, working on the data supplied by the above, and also by Milne-Edwards, Gray, Gunther, Leidy, Coues, and Dr. Peters, produced a complete system of cla.s.sification, which seems to be all that is to be desired.
We have already divided the rodents into two sub-orders, to which, however, Mr. Alston adds a third, viz., _Hebetidentati_, or Blunt-toothed Rodents, which contains only the _Mesotherium_, a fossil form. We have now to subdivide the two. The Double-toothed Rodents are easily disposed of in two families--_Leporidae_ and _Lagomyidae_. The Simple-toothed Rodents are more numerous, and consist of about eighteen families arranged under three sections, which are _Sciuromorpha_, or Squirrel-like Rodents, _Myomorpha_ or Rat-like Rodents, and _Hystricomorpha_, or Porcupine-like Rodents.
It would perhaps render it clear to the reader were I to tabulate the differences chiefly noticeable in these three sections:--
SECTION I.--SCIUROMORPHA, OR SQUIRREL-LIKE RODENTS.
Molar dent.i.tion 4--4/4--4 or 5--5/4--4. In the latter case the foremost upper molar is small; the fibula is distinct, and never united, except in some cases where it is attached to the extremity of the tibia; the zygomatic arch is formed chiefly by the malar, which is not supported beneath by a continuation of the zygomatic process of the maxillary; collar-bones perfect; upper lip cleft; the m.u.f.fle small and naked; tail cylindrical and hairy (except in _Castoridae_).
Five families.
SECTION II.--MYOMORPHA, OR RAT-LIKE RODENTS.
Molar dent.i.tion from 3--3/3--3 to 6--6/6--6, the former being the usual number; the tibia and fibula are united for at least a third of their length. The zygomatic arch is slender, and the malar process rarely extends so far forward as in the preceding section, and is generally supported below by a continuation of the maxillary zygomatic process; collar bones are perfect (except in _Lophiomyidae_); upper lip and m.u.f.fle as in the last; tail cylindrical, sometimes hairy, but commonly covered with scales arranged in rings. Seven families.
SECTION III.--HYSTRICOMORPHA, OR PORCUPINE-LIKE RODENTS.
With one exception (_Ctenodactylus_) have four molars in each upper and lower jaw; the tibia and fibula are distinct in young and old; the zygomatic arch is stout, and the malar does not advance far forward, nor is it supported by the maxillary zygomatic process; collar-bones perfect in some; the upper lip is rarely cleft; the m.u.f.fle clad with fine hair; tail hairy, sub-naked or scaly.