MAMMALIA. 73 



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. The ears are smaller, and the tail 

 is shorter, and less densely clothed with hairs than in Reithrodon cuniculoides. 

 The skull (see Plate 43, 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 

 fossaa are very shallow, whereas in R. cuniculoides they are deep. In the skull 

 of the animal just mentioned there are two distinct longitudinal grooves on the 

 palate, which extend backwards 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 fossae. At the bottom of each of these hollows are several minute 

 foramina, and in front of them there are two larger longitudinal foramina. In 

 R. chinchilloides, the longitudinal grooves on the palate and the posterior hollows 

 are shallow, and consequently much less distinct ; the pterygoid fossae are very 

 nearly on the same plane as the palate, and are indicated only by a very slight 

 depression. The incisor teeth are broader than in R. chinchilloides, and the molar 

 teeth are proportionately smaller. The thin plate which forms the anterior root 

 of the zygomatic arch is deeply emarginated in front in R. cunicidoides (see 

 Plate 34, fig. 21, b.) ; but in R. chinchilloides, the anterior margin of this plate is 

 nearly straight, (see Plate 34, fig. 20, c.) 



In the form of the lower jaw of the two animals under 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. cuni- 

 culoides, and that is, that the condyloid process is rather deeply concave on the 

 inner side, a character which does not exist in R. chinchilloides, nor do I recollect 

 having observed it in any other Rodent. 



The principal dimensions of the skull of R. chinchilloides, are as follows: — 



In. Lines. 



Total length 12 



Width 8| 



Length of nasal bones ............ 6^ 



of incisive foramina .... ( . ...... 4 



Distance between the outer surface of the incisors and the first molar tooth, upper jaw . 4£ 



Longitudinal extent of the three molars of the upper jaw, taken together . . . 2f 



Length of a ramus of the lower jaw without the incisor ...... 8 



