Mr. G. R. Waterhouse on the Rodentia. 199 



The small opening in the superior maxillary bone of the Sciu- 

 ridae forms only a passage for the infra-orbital nerve, which 

 in most other Rodents passes through together with the por- 

 tion of the masseter muscle before alluded to. In the Hystri- 

 cine section of Rodents (the Hystricidae, Echimyida, Caviidce, 

 &c. &c.) the ant-orbital opening is very large, whilst in the Mu- 

 rine group it is moderate or small, opens obliquely upwards, 

 and is almost separated from the opening for the nerve, which 

 passes through a narrow slit situated below the opening for 

 the muscle, though joining it : here we have a condition 

 which is intermediate between the structure observable in the 

 Sciu?'id(S and the Hystricine section. 



It is in the Murine group that we find a tendency to car- 

 nivorous habits, and it is in the groups of Rodents following 

 the Murina that we find the most truly herbivorous species, 

 accompanied often by an inferiority of powers in the extre- 

 mities. In fact, we find the most ruminant-like Rodents in 

 the Hystricine section, the most carnivorous species in the 

 Murida, and the most monkey-like in the Sciuridce. Thus, in 

 the Rodentia we find analogues at it were, or representatives 

 of other great divisions of mammals*. 



The Leporidce afford the greatest contrast to the Sciuridcs 

 observable : here the portion of the skull devoted to the 

 protection of the brain is very small, and the facial portion 

 large; the palate is most imperfect and leaves exposed the 

 sphenoids, the bodies of which, instead of being large and 

 expanded as in the Squirrels, are remarkably contracted, and 

 the openings for the transmission of the nerves are large. In 

 the large orbits, separated but by a narrow bony septum, the 

 large and united optic openings, imperfect palate, and narrow 

 bodies of the sphenoids, we are strongly reminded of the bird's 

 skull. 



In the Caviidce and Chinchillidce we find the nearest ap- 

 proach to the structure of skull and dentition observable in 

 the Leporida ; there is nevertheless much difference ; this pre- 

 sent family is in fact the most isolated among the Rodents. 



As regards the families, defined in the e Magazine of Na- 



* A similar system of representation is frequently observable in natural 

 groups. It is so strongly marked in the Marsupialia, that it has given rise 

 to the belief with some naturalists that that is not a natural group, but is 

 made up of members of other orders. I regard it as an error of the same 

 kind which has led to the separation of the Sloths from the Edentata, and 

 the grouping of those animals with or near to the Quadrumana, and the se- 

 paration from the last-mentioned group of the Galeopitheci and the Cheiro- 

 mys — placing the former with the Cheiroptera, and the latter with the Ro- 

 dents, 



