156 



and more contracted in front, and the palate is also more contracted 

 in front. The molar teeth of the upper jaw have the inner fold of 

 enamel deeper. In the lower jaw the molar teeth have the lateral 

 angles more produced, and tlieir transverse diameter is conseąuently 

 greater in proportion. The coronoid process is distinctly larger in 

 proportion. Other difFerences of size and proportion will be per- 

 ceived upon comparing the following dimensions : — 



O. Cuminyii. O. Bridgesii. 



in. lin. in. lin. 



Totai length of cranium 1 6| 1 9| 



Greatest width O 10| 1 0^ 



Length of nasal bones O 7 O 8į 



Length of frontai bones O 6į O 6# 



Width of interorbital space O 5 O 4\ 



Totai length of zygomatic arch O 8^^ O 11 



Length from front of superior incisors to the 1 rj c i f) fi * 



molar teeth J ^ ^ 



Length of the four molar teeth taken together O 4i 5^ 



Width of incisor teeth of upper jaw O IJ O 1;| 



Width of palate between foremost molars . . O 1^ O 1| 



Width of palate betvveen hinder molars .... O 2 O 2§ 



Length of ramus of lovver jaw O ^ ^3 1 If 



Height of ditto in a vertical line, dropped 1 q c 2 7 



from the condyle / ^ 



Mr. Waterhouse observed, that the skull in the genera Octodon 

 and Schizodon difFers from that of the nearly allied genera of Abro- 

 coma and Poephagomys, as well as the Echymys group, in having a 

 small vertical plate of bone which rises from the upper surface of 

 the anterior root of the zygomatic arch, and ■vvhich serves to protect, 

 externally, the infra-orbital nerve. The superior incisor tooth enters 

 the superior maxillary bone, and passes beyond the intermaxillary 

 suture by about one-sixth of the •whole length of the tooth ; whilst 

 in Abrocoma the incisor is shorter, terminating at the suture men- 

 tioned, and thus approaches the genus Lagotis, as well as in several 

 other characters which he had before noticed. Poephagomys is re- 

 markable for having the superior incisor tooth extended backwards 

 and outwards, covered by a thin fold of bone, and terminating on 

 the outer surface of the palatai portion of the skull, close to the third 

 molar tooth. 



Notwithstanding the great superficial resemblance which exists 

 between these animals and the MuridcB, it will be evident upon 

 examination that they belong to a different section of the Rodent 

 order, a section the species of which is readily distinguished, as he 

 had elsewhere pointed out, by the structure of the skull and lo\ver 

 jaw ; it is not, however, in these parts alone that difFerences exist 

 between the Octodontidee and the Muridce, for there is a dissirailarity 

 in the form of the muzzle, which he should take an early opportu- 

 nity of showing by means of drawings and desciiptions, made either 

 from the living animals or from specimens preserved in spirit, and that 



