10 DESCRIPTIONS OF PREPARATIONS. 



late Mammalia ; whilst others may have their connecting character expressed 

 by saying that the distinctive peculiarities of the Rodent type are not so 

 sharply pronounced in this as in the other suborder already mentioned, or 

 indeed in either of the two other suborders of Glires, the Hystricomorphi 

 and the Sciuromorphi. 



Among the last of these three sets of peculiarities may be mentioned 

 the shape of the articular surface furnished by the squamous bone for the 

 lower jaw. This surface is transversely, not, as usually in Rodents, antero- 

 posteriorly elongated ; and it permits consequently of a much greater 

 lateral movement of the jaw, correlated with which we find the molars 

 above and below not with horizontal but with concave and alternately 

 sloping grinding surfaces. The presence of six incisors in the upper jaw of 

 young, and of four in that of adult Lagomorphi^ is a third, the smaller 

 size of the sockets for the lower incisors a fourth, point indicating less 

 specialization in this suborder. 



In the relatively small extent to which the temporal muscle is de- 

 veloped, in the great extent to which the lower jaw is developed behind the 

 plane of its articular process, in the presence of a diastema between the 

 anterior scalpriform teeth and the molar series, and in the keel-shaped prae- 

 sternum prolonged into the cervical region as a ' proosteon,' the Lagomorphi 

 and most if not all other Rodents resemble many Ungulata, both Artio- 

 dactyle (such as Sus) and Perissodactyle (Equus and Tapirns). In the 

 reduction of the independence and importance of the fibula the Lagomorphi 

 and the Myomorphi resemble each other and many or most Ungulata, and 

 differ from all other Rodents, with some apparent exceptions, e. g. Pteromys 

 and Castor. In the length and slenderness of a process given off by the 

 squamosal posteriorly to the articular surface furnished by it to the lower 

 jaw, which process not being anchylosed, as in many other mammals, to the 

 tympano-periotic, nevertheless clamps it into fixed relations with the other 

 skull-bones adjacent to it, the skulls of the Lagomorphi and many other 

 Glires resemble those of some Perissodactyle Ungulata, whilst the presence 

 of a third femoral condyle, and of an internal alisphenoid canal for the 

 external carotid artery, are points in which they strikingly resemble all 

 living Perissodactyla. On the other hand, a curious illustration of the com- 

 bination in these Rodents of peculiarities which become separated in other 

 divisions of the class Mammalia is furnished to us by the ischium of the 

 Lagomorphi) which closely resembles the ischium both of the Ruminant and 

 non-Ruminant Artiodactyles in what is considered to be a distinctive pecu- 

 liarity of at least the latter of these two divisions of animals, viz. in the 

 presence on the outer side of the bone a little way in front of its upper and 

 posterior angle of a well-marked outstanding forwardly -curving process of 

 bone. The exposure in the dry skull of the turbinated bones in the nasal 

 cavity by the deficient ossification of the lateral walls of that chamber is 



