ORIGIN AND DEVELOPMENT OF THE EXISTING HORSES. 725 



in my judgment be sought for, in their large size, and possession of powerful 

 canine teeth which would grant them greater immunity from the attacks of their 

 fierce carnivorous contemporaries. With these means of defense they could take 

 up their abode in pastures where food more congenial to their tastes was furnish- 

 ed ; hence we can with perfect consistency look for a rapid modification of these 

 organs accompanied with slight change in others. In order to make the connec- 

 tions complete between them and the Phenacodonts there should yet be found an 

 Amblypod with bunodont molars reduced canines, and a more elongated foot. 

 An approach to this condition as far at least as the molar teeth are concerned is 

 •found in a new genus recently described by Prof. Cope under the name Manteo- 

 don (prophecy tooth;. The Phenacodonts present considerable variety of struc- 

 ture as far their anatomy is at present known. Prof. Cope has described five 

 genera. One genus, CatathlcBUs^ shows a peculiar sculpturing of the outside of the 

 molar teeth similar to that seen in many reptiles, and is the only mammal known 

 to possess it. In the genus Ajiacodon the molar teeth lack distinct tubercles, a 

 character which assigns it the lowest position 

 in the family. Phenacodus on the other hand 

 approaches nearest to the Lophiodonts in dent- 

 al characters, and is here taken for illustration, ^^c- 

 The teeth are forty- four in number, and are 

 disposed as follows : Incisors 4zf , Canines 

 -\Z\, Premolars |zA, Molars |zf . Fig. 4 rep- 

 resents a superior molar of a species of this Fig. 

 genus; a^ is the anterior external,/^ the pos- Fig. 4.-L eft sup. molar of a species of /';i^«- 

 terior external, ai the anterior internal and «^''^«^- Natural size; >- is drawn too large. 



. . . (After Cope). 



// the posterior internal lobes respectively. Fig. 5.-Left inf. molar of same. Natural 



They are low, obtuse and constitute the princi- size. (After Cope, 

 pal cusps of the crown, ace and pec are the anterior and posterior cross crests 

 which are rudimentary and represented by isolated tubercles in this animal, but 

 which are developed into important structures in the more specialized genera. 

 y is the rudimental external rib separating the antero and postero-external 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. In the in- 

 ferior molars (Fig. 5) the four principal cusps hold the same relation to the crown 

 and are lettered as above, z represents a low, indistinctly marked ridge, passing 

 from the postero external to the antero internal cusps /^ 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 ex- 

 ternal cusp pe. 



The feet as already stated possess five toes upon each foot. Fig. 6 exhibits a 

 diagram of a left anterior foot in which the third digit M Hi is the largest. The 

 second and fourth J/// and M iv are about equal to each other in size but small- 

 er, while the first and fifth are unequal and still more reduced. 



