46 SCOTT. [Vol. V. 



" tuberculo-sectorial," and consisting of an anterior triangle 

 composed of the proto-, para-, and metaconids, and a low pos- 

 terior heel or talon. The paraconid is much reduced, and in 

 some species cannot be distinguished. The talon is more or 

 less distinctly basin-shaped, and two cusps (the hypo- and ento- 

 conids) can be observed in it ; on the last molar there is an 

 additional median lobe, which represents the fifth cusp so uni- 

 versal among the artiodactyls. No other known artiodactyl 

 (except the problematical genus LeptocJicerus, Leidy) has such a 

 primitive molar dentition as this. There is nothing to distin- 

 guish it from some of the Condylarthra, the pseudo-lemuroids, 

 or even certain creodonts, which is to say that it has still pre- 

 served the plan common to all these groups — a fact of the 

 highest morphological interest. 



In view of the fact first pointed out by Schlosser (So. 30, p. 

 267), that the premolars even when entirely like the molars in 

 structure, have passed through an entirely distinct course of 

 development, and that the similar parts in the two series are not 

 homologous, the premolars of Pantolestes deserve careful exami- 

 nation. Of the superior series only the fourth is known, and 

 this is entirely like the corresponding tooth of most creodonts, 

 consisting of a single compressed and trenchant external cusp, the 

 protocone, and a conical internal cusp, which may be called the 

 deuterocone. It is important to remember that the protocone is 

 external in position, while in the superior molars it is internal. 

 The inferior premolars are all simple, compressed, and trenchant 

 cusps, implanted, except the first, by two fangs. More or less 

 distinct anterior and posterior basal cusps are present, and in 

 one specimen of P. brachystomus I find in the fourth premolar 

 of one side (but not of the other side) a distinct deuteroconid 

 occupying the same position as in the supposed creodont genus 

 Chriacus. 



The second member of the series is the genus Homacodon, 

 Marsh, from the Bridger, which presents us with a great advance 

 in the dentition, in that the molars have become quadrate in 

 outline and quadrituberculate in structure. In the upper series 

 this change is brought about by the addition of the hypocone ; 

 strictly speaking, these molars are sexituberculate ; for though the 

 metaconule has become larger, the protoconule is frequently, if 

 not generally, present as well, though evidently about to dis- 



