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Show MAMMALIA. probable, therefore, that the colours have been slightly changed. It is of a smaller size than either of the preceding species. Its fur is long, extremely soft, and somewhat resembles that of the Chinchilla. Tile ears are smaller, and the tail is shorter, and less densely clothed with hairs than in Reithrodon cunicutoides. The skull (see Plate 4.'3, fig. 20, a, 20, b, and 20, c,) differs in many respects from that of the species last mentioned. It is of a smaller size, the nasal portion is proportionately shorter and narrower, the incisive foramina are shorter; the pterygoid processes do not approximate so nearly at their base, and the pterygoid fossr:e arc very shallow, where as in R. cnniculoides they are deep. In the skull of the animal just mentioned there are two distinct longitudinal grooves on the palate, which extend back wards from the incisive foramina, and terminate in two rather large and deep excavations : these excavations are in the palatine bone, and situated between the last molar teeth; they are separated from each other by a narrow, longitudinal, elevated ridge; a narrow ridge also separates them from the pterygoid fossr:e. At the bottom of each of these hollows are several minute 1oramina, and in front of them there are two larger longitudinal foramina. In R. cllincltiltoides, the longitudinal grooves on the palate and the posterior hollows are shallow, anti consequently much less distinct; the pterygoid fossre are very nearly on the same plane as the palate, and ar·e indicated only by a very slight depression. The incisor teeth are broader than in R. cltinc!tilloides, and the molar teeth are proportionately smaller. The thin plate which forms the anterior root 73 of the zygomatic arch is deeply emarginated in front in R. cuniculoides (see Plate 34, fig. 2J, h. ) ; but in R. cltincltilloides, the anterior margin of this plate is ueal'1y straight, (see Plate 34, fig. 20, c.) In the form of the lower jaw of the two animals nuder consideration there are differences which will be more clearly understood upon comparing the figures. I will therefore merely notice one remarkable character which is found in R. cuniculoides, and that is, that the condyloid process is rather deeply concave on the inner side, a character which does not exist in R. cltincllilloides, nor do I recollect having observed it in any other Rodent. The principal dimensions of the skull of R. cltincltilloides, are as follows:- Totnl length Wiclth Length of nasal hones of incisive fommina . . • Distance between the outer snrfnco of tho incisors nnu the first mohr tooth, upper jaw Longitudinal extent of the three molars of the upper jaw, taken together Length of a ramus of the lower jnw without the incisor L ln. I 0 0 0 0 0 Lines. 2 8~ GA 4 4~ 2..i . 8 |
