Zoological Society. 479 



marked in the Hogs, which serves, as I have found in the Sheep, for 

 the origin of the obhquus inferior muscle of the eye, the remainder 

 of the fossa being filled up with adipose matter. In the Perissodac- 

 tyla no such fossa exists, and there is never more than a very slight 

 depression marking the origin of the muscle, in most cases not per- 

 ceptible at all. The shortening of the bony palate in the latter group, 

 the small diiference of level between it and the base of the cranium, 

 together with the longitudinal extension of the posterior nasal orifice, 

 the lateral spreading-out of its walls and the constant existence of the 

 alisphenoid canal, which I pointed out in my former communication, 

 may be again adverted to. 



The pterygoid ridge in this group is not very strongly marked, and 

 gradually dies away upon the lamina enclosing the alisphenoid canal ; 

 the pterygoid processes have considerable antero-posterior extent, and 

 the true pterygoid bones are reduced to mere ribands. On the other 

 hand, in the Artiodactyla, the pterygoid ridge, continued from the in- 

 ferior root of the zygoma, terminates abruptly, with a free process in 

 the Ruminants ; while in the Hogs and other allied forms, it is from 

 this process that a laterally projecting plate extends down on the outer 

 side of the pterygoid process, forming a pterygoid fossa in a manner 

 diiferent from all other mammalia, and very characteristic of these 

 Non-ruminant Artiodactyles. The temporal bone in the Perissodactyla 

 also furnishes characters in the back of the zygoma, which gently 

 slopes away to its origin, and in the association of a distinctly marked 

 eminentia articularis vAth. a rather large and more or less thickened 

 and mamilliform post-articular process. The principal differences 

 in the occipital bone I pointed out in my former paper, and notwith- 

 standing the marked difference between the Hog and the Ruminant, 

 I must observe that they agree in the flatness and squareness of the 

 basal portion, while in the Perissodactyla it is transversely convex, 

 being rounded off on each side into the great foramen lacerum. 



I mentioned in a note appended to my former communication, an 

 idea which occurred to me just before that paper went to press, that 

 a further distinction between the two groups might be found in the 

 structure of the premolar teeth. I have found, on investigation, that 

 the character will not always admit of being rigidly applied, since in 

 some genera of Perissodactyla, as the Lophiodon to which I there 

 alluded, the posterior lobes of the premolars are not so completely 

 developed as they are in the true molars ; and on the other hand, in 

 some of the Artiodactyla, as the Peccary, they advance a little beyond 

 the rudimentary condition in which they are usually found, though 

 never attaining an equal development with the others. The character 

 will however in most cases enable us to distinguish ; and in the course 

 of the observations I was thus led to make, I have discovered another 

 more important one, which I will next proceed to explain. 



If we consider as an entire molar tooth that which has four prin- 

 cipal tubercles, the molars of the lower jaw must be said to be placed 

 each in advance of its homologue in the upper jaw to the extent of a 

 quarter of a tooth, so that the premolars, which in most cases repre- 

 sent but half molars, alternate with their opposing teeth above. 



