480 Zoological Society. 



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 pretypify 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 are supernumerary, since they close between 

 the two pair of principal tubercles of the penultimate upper milk 

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

 the penultimate lower milk molar, which in this as in most groups 

 represents but the half of a true molar, furnishes opposition to its 

 most anterior surface. Although it is not always literally true, that 

 in the Artiodactyla the premolars represent each but the half of a 

 true molar, and in the Perissodactyla an entire one, it is certain 

 that in the exceptional cases among the former group, the parts re- 

 presenting the posterior division of the tooth are small, or merely 

 rudimental ; and that in the latter group, it is only in the most an- 

 terior of the series that the posterior portion of the tooth is ever 

 altogether wanting. It is also certain, that all those genera of which 

 the milk dentition has been seen, conform in that particular to the 

 general character, the distinction being well-marked in the Artio- 

 dactyla between the two last upper milk teeth, whose characters are 

 those of true molars, and those which precede them and represent but 

 half ones, the same difference also prevailing between the last and 

 those which precede it in the lower jaw ; always necessitating the ex- 

 istence of a third pair of tubercles in the last lower milk molar to 

 work in the interval of the two pairs in the penultimate above ; while 

 in the Perissodactyla, the constant existence of a well- developed pos- 

 terior pair of lobes in the penultimate lower milk tooth abrogates the 

 necessity of a third pair in the last one, and consequently we need 

 not expect to find it, even in those genera, such as Lophiodon and 

 Palceotherium, of which the additional lobe to the last true molar is 

 characteristic. Of the first-named genus, the milk dentition, so far 

 as I am at present aware, is as yet unknown ; but among the plates 

 in the *Ossemens Fossiles' examples may be seen of the lower jaws of 

 young Palseotheria, exhibiting the milk teeth, of which the last has but 

 two lobes*. Therefore the tripartite condition of this tooth becomes 

 a constant and important character of the Artiodactyle division. 



Most of the characters which separate the Ruminant and Non-ru- 

 minant divisions of the Artiodactyla have been pointed out in my 

 former paper, as well as those which distinguish the two subdivisions 

 of the Hog-tribe, which by the analogy of the amount of difference in 

 those of other groups, I think must be looked upon as families, — 

 SuidcB and Hippopotamidce. The striking character derived from the 

 sudden termination of the pterygoid ridge in the Ruminant, and the 

 formation of the pterygoid fossa in the other division, has been alluded 



♦ PI. 4. fig. 1 (alluded to by Professor Owen), and pi. 56. fig, 2. 



