Copp.J i'c5 [Dec. 16, 



bunoclont or suilline division of the Artiodactyla. In this character, how- 

 ever, there is a closer gradation than in the case of the Artiodactyla, and it 

 would scarcely be necessary to create such a group on that character alone. 

 But the genus differs further from the Perissodactyla and approaches the 

 Proboscidia, in the fact that the astragalus articulates with the navicular 

 only, and by a universally convex surface, as in the Carnivora. 



The astragalus resembles that of the latter order very closelj-, and difiers 

 from that oi Hyvaeotherium and the nearest forms among tlie Perissodactyla. 

 Phenacodus has moreover five well developed toes on all the feet, and was 

 probably not entirely plantigrade. The cast of the brain case shows that 

 the cerebral hemispheres were quite small and nearly smooth, and that the 

 very large cerebellum and olfactory lobes were entirely uncovered by 

 them. The bones of the two carpal rows alternate with each other, and 

 there is a large third trochanter of the femur. The cervical vertebrae are 

 opisthocoelous. 



This group is then the ancestral tj'pe of the known Perissodactyla, that 

 is of the horses, tapirs and rhinoceroses, and of the numerous extinct 

 forms. Its systematic position may be schematically represented as 

 follows : 



Order Perissodactyla ; ungulate ; digits of unequal lengths ; carpal 

 bones alternating ; a postglenoid process. Astragalus wiih proximal 

 trochlea, and witliout distal double ginglymus. 



Suborder Diplarthra ; astragalus distally plane or concave in one direc- 

 tion, and uniting with both navicular and cuboid bones : a -third trochanter 

 of the femur. The known families belong here. 



Suborder Gondylarthra ; astragalus convex in all directions distally, 

 only uniting with navicular bone ; a third trochanter of femur. 



Family PhenacodontidoB. Molar teeth tubercular ; the premolar teeth 

 different from the molars ; five digits on all the feet. 



Genera ; Phenacodus Cope, and very probably Catatldmis, A7iacodon and 

 Protogonia Cope, and perhaps also Anisonclius Cope. These genera include 

 fifteen species, all from the lower Eocene beds. I gave a synopsis of their 

 differential dental characters in the Proceedings of the Philosophical Society, 

 1881, p. 487, where I included also the genus 3Bocl(enus. I omit the latter 

 from the family at present, as I believe it to be Artiodactyle. 



Phenacodus prim^vus Cope. 



Parts of a dozen individuals of this species were obtained, and one 

 almost entire skeleton in a block of soft sandstone. This includes nearly 

 all parts of the four extremities, as well as the skull, from which but small 

 portions are wanting. 



Species of this genus, so far as determinable from the dentition, are 

 numerously represented in Mr. Wortman's collection. About fifty individ- 

 uals are referable to eight species. These present a great range in size, 

 and some diversities of structure. They may be distinguished as follows : 



