362 



MECHANICAL EVOLUTION. 



inferior sectorial to a primitive five-tubercled, or '^tubercular 

 sectorial" type. Farther than this I did not go, and made no 

 attempt to derive the few cases of triangular superior molars then 

 known, nor the type of the superior sectorial. The revelations of 

 the Puerco fauna show that the superior molars of both ungulate 

 and unguiculate Mammalia have been derived from a tritubercular 



Fig. 65. — Deltatherium fnndaminis Cope, skull and ramus mandibuli, two thirds 

 natural size, from the Puerco beds of New Mexico. Fitrs. a, i, c, from one individual. 

 Fig, d^ from a second animal. Fig. a, right side of cranium ; J, palate from below ; c, 

 mandible part, from above ; d^ left ramus, outer side. From the Eeport U. S. Geol. 

 Surv. Terrs., vol. iii. 



Original. 



type ; and that the inferior true molars of both have been derived 

 from a " tubercular sectorial " type. Shall we look for the origin 

 of the latter in a tritubercular tooth also, i. e., tubercular sec- 

 torial without heel ; and will the crowns of the true molars of the 

 primitive mammals alternate with, instead of opjDose each other ? 

 This is a probable result of future discovery. 



