In the beaver the fibula gives off' from its upper extremity a strono. recurrent apophysis, which is directed slightry outwards. In some genera the fibula is excessively slender, and does not reach so low down as to become connected with the lower extremity of the tibia.
In those Rodents which have the fibula con solidated with the tibia towards its tarsal ex tremity, the tarsus seems to he articulated with the latter bone only ; but if very young individuals are examined carefully, is per ceptible that the external malleolus is formed by the fibula.
In Rodentia the os calcis is very much elorg ted posteriorly.
In such genera as have five complete toes the following circumstances may be remarked : — In the beaver the os scaphoides is divided into two portions, one placed in front of the astragalus, which supports the second and third cuneiform bones, and one placed inter nal to the astragalus, to which is attached the cuneiform bone that supports the great toe, 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 nurnerary bone. Among „those genera which have only four toes, such as the helamys, or Cape jerboa, which has its foot exceedingly elowmted, the inferior tubercle of the scaphoid, whicl 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 phalanns of the toes.
In the rabbit and the hare, animals which resemble–the jerboa in the great size of the tubercle of the seaphoid, 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 disp–osition 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 pins jaculus) and the alactaga plus 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 dentes 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, vvhich 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 rnasseter muscles. The molar teeth are like wise exceedingly strong, and vary considerably in their mode of implantation in the jaws of different genera.