ORIGIN AND DEVELOPMENT OF THE EXISTING HORSES. 69 



exhibit a stiM more complicated pattern, and furnish a complete transition between 

 the inferior molars of Hyracotherium and the double crescents of the true PalcBo- 

 theroid type. This condition is accomplished by the greater development of 

 the antero-posterior crest z, and the higher cross crests, connecting the external 

 with the internal cusps as shown in the cut. The antero internal lobe ai is di- 

 vided into two distinct tubercles ai and ai' . The ridge k is strong and prominent. 

 It is important to notice in this connection that while the teeth of the lower 

 Eocene genera of this family {^Lambdotherium and Oligotomus, Cope) resemble very 

 strongly the teeth of the lower forms of the Lophiodonts in the shortness 

 of their crowns and approach to the Bunodont type, the latter genera possess 

 more lengthened cusps and simulate the selenodont forms in the crescentic 

 section of some of them. The connections .y., 



heiwQQn the ChalicotheriidcB a.nd Lophiodon- P\ ""^^M !f k^^^'f* Y^i 



... ^ W'^^1" we ■■ 3-^ 



tid(z are so close that it is indeed difficult to ^[ „„„ 



draw the dividing line between them. It pcc 



has been constructed, however, upon the . ,.^^^K/ ""■••■ai 



presence or absence of the external vertical ^ P'' P n 



rib of the superior molars, as already men- Fig. io. Fig. ii. 



tioned, but it is questionable whether this Fig. lo.— Right sup. molar of a species of z-aw*. 



character is of true family significance or '^^'^^"«'«' "^'- ^'^«' (^^'^ Cope). 



rj^, c ^ c 1 J ^ j-i^T Fig. 11.— Last inf. molar of left side of same, nat. 



not. Ihe feet as far as known do not differ size, (after Cope), 

 from those of Lophiodontidce. 



From this family, we pass to a consideration of the remaining links between 

 them and the horses proper, which is afforded by the FaliBotheriidce. Here the 

 premolars become more compUcated and assume the same structure as the molars 

 while the anterior limbs suffer a still further reduction of digits, leaving three upon 

 each, with the outer ones still more reduced (Fig. 14). In one genus, however 

 {Mesohippus, Marsh), the anterior limbs still retain a rudiment of the fifth meta- 

 podal. The structure of the true molars in the lower forms of the Palceotheriidce 

 is very little different from the higher genera of the Chalicotheriidce and in point 

 of dental specialization, the increased complexity of the premolars forms the only 

 marked difference. 



K '&\x^tx\ox tcloXzx oi Anchitherium 'y~> shown in Fig. 12. The four principal 

 cusps ae, pe, ai and pi, are considerably lengthened and are connected by high 

 ridges ace a.nd pec, passing in an obHque direction across the crown. The eleva- 

 tion of the cusps and crests gives increased depth to the valleys. The anterior 

 basal lobe is reduced and the external rib y is strong. The ctown is further com- 

 plicated by the addition of the lobe /. 



In the inferior molars (Fig. 13) the two crescents are complete and their 

 vertical dimentions augmented. The antero-internal lobe shows stronger separa- 

 tion, forming the lobe ai', and the anterior ridge k is well produced. 



