48 SCOTT. [Vol. V. 



molars are still very brachyodont and of the typical tetra-seleno- 

 dont character ; the median ribs of the external crescents are 

 still prominent, and the antero-external buttresses small. Pre- 

 molars i to 3 are simple, compressed, and trenchant cones, 

 much elongated antero-posteriorly ; pm. 4, on the other hand, has 

 assumed the structure which it has in all ruminants, consisting 

 of an internal and an external crescent. Apparently this tooth 

 corresponds to half a molar, but from what we have seen in 

 Pantolestes it is plain that the protocone is here the outer cres- 

 cent, while in a true molar it forms the antero-internal crescent, 

 and the deuterocone, which forms the inner crescent, has no 

 strict homologue in the molar crown at all. It is rarely the case 

 among Artiodactyla that the premolars become molariform, but 

 this does occasionally occur (e.g. Agriochcerus, Dicotyles) ; and 

 though the molar pattern may be fully attained, the elements 

 which compose the crown are differently arranged and are not 

 homologous with those which occupy similar positions in the 

 crowns of the true molars. The first step towards the assump- 

 tion of the molar pattern in the last upper premolar, is the addi- 

 tion of a second external cusp behind the protocone, which may 

 be called the tritocoue. This occupies the same place as the 

 metacone of the true molars ; but as its position with reference 

 to the protocone is entirely different, it cannot be given the 

 same name. The addition of the tritocone produces a trituber- 

 cular crown similar to that of the primitive molars, but with 

 the elements which compose it not homologous with those of 

 the molar crown. A second internal cusp, which may be called 

 the tetartocone, is next added, and this corresponds to the hypo- 

 cone of the true molars, and completes the molar pattern. In 

 Agriochcerus (Fig. B) this tetartocone is still very small, in some 

 specimens hardly noticeable. 



The steps of differentiation by which the inferior premolars 

 become molariform are not quite so clear, but they may be 

 made out in Agriochoerus and Dicotyles. Comparing pm. 4 of 

 Oreodon with that of Agriochcerus, we see that the first addition 

 to the protoconid is a cusp on the inner side, which may be 

 called the deuteroconid, and then the only further step which is 

 necessary to the assumption of the molar pattern, is the eleva- 

 tion of the talon and its division into hypo- and entoconids. The 

 almost perfectly molariform pm. 4 of this genus retains an inter- 



