/ 



Mr. G. R. Waterhouse on the llodentia. 345 



PAmcrique ne nourrit aucun Acrodonte." I may mention 

 that, according to the ' Histoire Naturelle dcs Reptiles/ the 

 section Pleurodontts contains thirty-one genera and ninety- 

 five species, one of which only is fonnd in the Old World ; 

 whilst the section Acrodontes contains fifteen genera and fifty 

 species, none of which are found in America. As instances 

 among birds, the Muscicapidce and Nectariniidce may be no- 

 ticed; in both these groups the New World species are di- 

 stinguishable from the Old by the structure of the wing*. 

 Very many similar cases might be recorded. 



Having determined upon a classification of the Rodents 

 (founded chiefly upon characters furnished by the skull and 

 lower jaw), I was not a little interested to find in that group 

 another illustration of this class of facts, — to find that a great 

 mass of the South American Rodents belonged to a section 

 which has but few representatives elsewhere, — the 



Hystricina. 



All the species of this section have four molars on either 

 side of each jaw; in those which are placed at the head of the 

 group the molars are rooted, in the remaining species they are 

 rootless. The skull is broad between the orbits; the ant- 

 orbital opening is always large ; the palate is usually con- 

 tracted, especially between the anterior molars, and deeply 

 emarginated behind. In the highest Hystricina (which have 

 rooted molars) the bony palate is less deeply emarginated be- 

 hind, and sometimes the molars are parallel, — a somewhat un- 

 common character in the present section ; the palatal open- 

 ings are small, and the bodies of the sphenoids are expanded 

 and w r ell-developed. Descending in the series the palate be- 

 comes less and less perfect, and the bodies of the sphenoids 

 are contracted, until in the lowest — especially in the Chin- 

 chillas {Chinchillidae) — we find a condition in these parts 

 closely approximating to the hares (Leporida). 



Although the number of molar teeth, combined with the 

 large ant-orbital opening to the skull, would generally serve 

 to distinguish the Hystricina from the Murina, there are a few 

 species of the last-mentioned section which exhibit these cha- 

 racters. To define the Hystricina, therefore, it was necessary 

 to seek for other points of distinction — these I have found in 

 the conformation of the lower jaw. 



The various modifications in the form of the lower jaw in 

 the Murina have already been pointed out ; and, accompany- 

 ing the present observations, I have given figures representing 



* I believe Mr. Swainson first noticed this fact. 



