149 



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 hare considerable antero-posterior extent, and 

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

 hand, in the Artiodactyla, tlie 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 

 different 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 

 emineutia articularis with a rather large and more or less thickened 

 and mammilUform 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 observ'e 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 commmiication, 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 x\rtiodactyla, 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. 

 It is in accordance with this universal law, that the last lower milk 

 molar in the Artiodactyle division of the Ungulata has three pair of 

 lobes ; not, as has been imagined, that it may prety])ify the last true 

 molar, which in the same group is usually also six-lobed. The last 

 lower true molar, being placed like the rest, a quarter of a tooth in 

 advance of its four-lobed opponent, the pair of tubercles that are added 

 to it behind play against the posterior surface of the hindmost pair of 

 lobes of the upper tooth ; but in the last lower milk molar it is the 

 anterior pair of cusps that arc supernumerary, since they close between 

 the two pair of princi|)al tubercles of the peiuiltimate uj)per milk 

 tooth, which like the last one has the form of a true molar ; while 



