OCR Text |
Show 84 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. In the Octodont1:dce the skull is rather short., the inter-orbital space is broad; the ant-orbital passage is large; the zygomatic arch is thrown out horizontally from the plane of the palate; the malar bone is broad and somewhat compressed, and throws up a small post-orbital process; the gleuoid cavity of the temporal bone is narrow; the palate is contracted, and deeply notched posteriorly, the portion which lies between the molar teeth descends below the level of the anteriot· portion; the incisive foramina are wide: the body of the auterior and posterior ~:;ph eno ids is very narrow, and the foramina on either side of them are large : the occipital condyles arc very narrow, widely separated, and the articular surface is nearly vertical.* The descending m1nus of the lower jaw springs ft·om the outer side of the alveolat· portion, and terminates in a point, more or less acute. The incisors or" the upper and lower jaws are of the same width : the molars arc t={, rootless. ln external characters the species of the present group vary considerably. The toes are 5 !5 or 4!5. The claws of the hind feet are covered by strong, curved bristly hairs. · The principal points of distinction in the external characters of the four genera under consideration, may be thus expressed. 1' TOES 5\5 . A . Fore feet formed for burrowing-strong and armed with large claws; tail short. a. Ears minute, incisors very broad b. Ears small , incisors broad B. F ore feet weak; claws small; incisors n[trrow; cars large. a. Tail with the apieal portion furnished with long hair H TOES 4·15. h. Tail furni shed throughout with short adprcssed hairs Gtenomy.Y. Poepltagomys. Octodon. Abrocoma. It is not only in the comparatively small size and weakness of the fore feet that Abrocoma approaches more nearly to Octodon; but it agrees in having the soles, both of the fore and hind feet (which are devoid of hair), covered with minute round fleshy tubercles (see the under side of the tarsus figured in Plate 28.) In Octodon, however, the toes have on their under side transverse incisions, as the Mu1'idce, and many other Rodents; a character not found in Abrocoma. * There is a wide difference between tho present animals aud the Arvicolidw in the form oC tho occipital condyles: tho same diftcrenec is also observable between Ecltirnys and Mus. The Octodontidw in fact l1ave the ~am e form of condyles as tho Chincl1illas and Cavies. In this and many other characters the last mentioned animals evince an affinity to tho Leporidw. |