TEETH FROM PHENACODUS TO EQUUS. 267 



1. Left upper molar of a species of Phenacodus, nat. size (Cope) ae 's 

 the antero-external, pe the postero-external. ai the antero-internal and pi 

 the postero-internal lobes respectively. They are low and obtuse and con- 

 stitute the principal cusps of the crown, ace and pec are the anterior and 

 posterior cross crests ; they are rudimentary and represented by isolated 

 tubercles in this animal, but are developed into important structures in the 

 more specialized genera, y (the lobe is drawn too large) is the rudimental 

 external rib separating the antero and postero-exterual cusps. An antero- 

 basal lobe arising as an outgrowth from the cingulum or ledge surrounding 

 the base of the crown is strongly marked in some genera. 



2. Left lower molar of same, nat. size, z represents a low, indistinct- 

 ly marked ridge, passing from the postero-external to the antero-internal 

 cusps pe, ai. The antero-internal cusp ai is sometimes double, h is the 

 heel, which is so strong in the last molar as to be called a fifth lobe. It is 

 connected by a faint ridge with the postero-external cusp pe. The four 

 principal cnsps ae, pe, ai, pi hold the same relation to the crown as in the 

 upper molar. 



3. Right upper molar, of a species of Lambdotherium, in which the an- 

 tero and postero-external cusps ae, pe are separated by an external vertical 

 ridge, y ; nat. size (Cope). 



4. Last lower molar (left side), of same ; nat. size. The antero-in- 

 ternal lobe is divided into two distinct tubercles, ai, ai'; the ridge k is 

 strong and prominent. The breadth of the tooth is accounted for by the 

 fact that it is the last molar, the first and last molars being about a third 

 broader than the others. The teeth are of a more complicated pattern than 

 those of Phenacodus. It is important to notice that while the teeth of the 

 lower Eocene genera of this family (Lambdotherium and Paleosyopous) re- 

 semble very strongly the teeth of the lower forms of the Lophiodons in the 

 shortness of their crowns and approach to the bunodont type, the latter 

 possess longer cusps and simulate the selenodont forms in the crescentic 

 section of some of them. 



5. Left upper molar of Anchitherium aureliauense, nat. size (Gaudry). 

 The four principal cusps ae, pe, ai, pi are considerably lengthened and con- 

 nected by high ridges, ace, pec, which pass in an oblique direction across 

 the crown. The elevation of the cnsps and crests give increased depth to 

 the valleys. The anterior basal lobe is reduced and the external rib y is 

 strong. The crown is further complicated by the addition of the lobe I. 



6. -Right upper molar of a species of mppoiherium. The valleys, which 

 are deepened by the lengthening of the cusps and ridges, are filled by a 

 thick deposit of cement, but the cement, as the cut shows, has been re- 

 moved. The points of the cusps and ridges are unworn. The four princi- 

 pal lobes ae, pe, ai, pi hold about the same relation to each other. The 

 cross crests acc,pcc have their obliquities increased, and the anterior bends 

 aronnd on the inner part of the face and becomes confluent with the pos- 

 terior ridge pee. The lobe I, which is conic in Anchitherium, is elongated 

 in a transverse direction to the crown, so as to close the posterior valley 

 and join the posterior external cusp pe with the posterior crest pee. Addi- 

 tional vertical pillars are developed on the cross ridges. The teeth resem,- 



