382 



RODENTIA. 



and a supernumerary flattened bone situated 

 along the inner margin of the tarsus. The same 

 disposition of these bones exists in the genera 

 spalax and capromys, in the marmot, squirrels, 

 and porcupines ; but in the four latter genera 

 the supernumerary bone is of smaller size. 



The rats and the paca have their os sca- 

 phoides divided, but are without any super- 

 numerary bone. Among j.those genera which 

 have only four toes, such as the helamys, or 

 Cape jerboa, which has its foot exceedingly 

 elongated, the inferior tubercle of the scaphoid, 

 which is observable in the sole of the foot of 

 all rodents, is very long and prominent. Upon 

 the internal margin of the tarsus there are in 

 this animal some elongated flat bones, which 

 are the rudiments of the great toe. 



In the jerboas, properly so called, both the 

 internal and external metatarsal bones are ex- 

 tremely small, and the three others are con- 

 solidated into one bone, upon the distal ex- 

 tremity of which are three articulating surfaces 

 which support the phalanges of the toes. 



In the rabbit and the hare, animals which 

 resemble the jerboa in the great size of the 

 tubercle of the scaphoid, the rudiments of 

 the great toe become consolidated at an early 

 age with the metatarsal bone of the second 

 toe. 



In the capybara, the Guinea-pig, the mara, 

 and agouti, which have only three toes, the 

 internal portion of the scaphoid supports a 

 single bone, representing the cuneiform and a 

 rudiment of the inner toe ; the cuboid like- 

 wise supports a small bone, which is a ru- 

 diment of the outer toe. The disposition of 

 the toes varies considerably in the different 

 genera of Rodents ; in the beaver, the inner 

 toe is nearly of equal length with the others ; 

 in the marmot, the porcupine, and the rats, 

 it is considerably shorter ; in the paca it is 

 nearly obliterated ; and in the Cape jerboa it 

 is a mere rudiment, consisting of but a single 

 bone ; in the hares not even this rudiment is 

 perceptible. 



In the capybara, the agouti, and the 

 Guinea-pig both the inner and outer toes are 

 reduced to a single bone. 



The jerboa (Musjacu/iis) and the alactaga 

 (Mus sagitta) have their three middle meta- 

 tarsal bones consolidated into one piece. 

 The two lateral toes are distinct in the jerboa, 

 but of comparatively small size ; in the alac- 

 taga they are quite wanting. 



Teeth. The distinguishing character of 

 the order of quadrupeds under consideration 

 is the remarkable arrangement of their dental 

 system, by which they are enabled to erode 

 the hardest vegetable substances. The chief 

 food of many genera, indeed, consists of the 

 bark, wood, and even the hard fruits of trees, 

 to devour which necessarily requires great 

 strength of jaw, and such a disposition of 

 their incisor teeth as to convert them into 

 strong chisel-like cutting weapons, the edges 

 of which never become blunted even to the 

 latest period of life. 



These incisor teeth, called also denies 

 scalprarii, are situated in the front of the 



mouth," and are generally two in number in 

 each jaw, except in the genus Lepus, em- 

 bracing the hares and rabbits, which possess 

 two small additional incisors, situated behind 

 each of the large ones contained in the upper 

 jaw. 



Between the incisors and the molar teeth 

 there is a considerable vacant space, by which 

 arrangement the play of the anterior chisels 

 is much facilitated, their action being like- 

 wise materially assisted by the mode of arti- 

 culation of the lower jaw, which allows of 

 considerable movement from behind forwards, 

 and by the great power of the pterygoid and 

 masseter muscles. The molar teeth are like- 

 wise exceedingly strong, and vary considerably 

 in their mode of implantation in the jaws of 

 different genera. 



The incisors* are always regularly curved, 

 the upper ones describing a larger segment 

 of a smaller circle, the lower ones a smaller 

 segment of a larger circle; these are the longest 

 incisors, and usually have their alveoli ex- 

 tended below or on the inner side of 

 those of the molars to the back part of the 

 lower jaw. Like the molars of the Me- 

 gatherium, and other teeth of unlimited 

 growth, the implanted part of the long and 

 large incisors retains the form and size of the 

 exposed part or crown to the widely open 

 base, which contains a long, conical, persistent 

 dentinal pulp, and is surrounded by the cap- 

 sule in a progressive state of ossification as it 

 approaches the crown, an enamel pulp being 

 attached to the inner side of that part of the 

 capsule which covers the convex surface of 

 the curved incisor. The matrix is here no- 

 ticed in connection with the tooth, because it 

 is always found in full development and 

 activity to the time of the Rodent's death. 

 The calcification of the dentinal pulp, the de- 

 position of the earthy salts in the cells of 

 the enamel pulp, and the ossification of the 

 capsule proceed contemporaneously ; fresh 

 materials being added to the base of the vas- 

 cular matrix as its several constituents are 

 progressively converted into the dental tissues 

 in the more advanced part of the socket. 

 The tooth thence projecting consists of a 

 body of compact dentine, sometimes with a 

 few short medullary canals continued into it 

 from the persistent pulp cavity, with a plate 

 of enamel laid upon its anterior surface, and 

 a general investment of cement, which is very 

 thin upon the enamel, but less thin in some 

 Rodents, upon the posterior and lateral parts 

 of the incisor. The substance of the incisor 

 diminishes in hardness from the front to the 

 back part of the tooth ; the enamel consisting 

 of two layers, of which the anterior and ex- 

 ternal is denser than the posterior layer, and 

 the posterior half of the dentine being by a 

 modified number and arrangement of the 

 calcigerous tubes less dense than the anterior 

 half. 



The abrasion resulting from the reciprocal 

 action of the upper and lower incisors pro- 



* Owen, Odontograpliy, p. 398. 



