1910.] Characters of the Stem Perissodactyls. 391 



ricular meatus (arch) wide and well defined by large postglenoid and post- 

 tympanic processes of the squamosal; the latter confluent with the down- 

 ward extension of the mastoid and this in turn with the downwardly directed 

 paroccipital process of the exoccipital. Mastoid a wedge-like bone showing 

 on the oblique postero-external corner of the occiput. Post-mastoid fora- 

 men present. 



The preceding skull characters may confidently be assigned to the stem 

 Perissodactyl and are all inherited from an Insectivore-Creodont-Condylarth 

 plan. 



Vertehroe in essentials similar to those of Phenacodus. Atlas with para- 

 pophysis pierced by vertebraterial artery. This character is lost in the 

 Oligocene Titanotheres (Osborn). Axis with peg-shaped odontoid. Pleura- 

 pophysial flange of sixth cervical vertebra very broad, obliquely placed. 

 Lumbars with transverse processes directed slightly forward. Sacrals at 

 first probably of Creodont-Condylarth type, namely, transverse process of 

 anterior sacral widely expanded and forming the chief support of the pelvis, 

 those of second and third sacrals rapidly narrowing. Later the second and 

 third sacrals became broader and shared in the iliac attachment, at the same 

 time the transverse process of the fourth, fifth and sixth sacrals broadened 

 but in decreasing degrees, producing the characteristic Perissodactyl type 

 of sacrum. Dorso-lumbar formula possibly 20 or 21 (15 or 16 + 4-6). 

 In correlation with the increasing size of the viscera and thorax, the number 

 of rib-bearing vertebrte increase. The primitive Condylarth number 

 (19-20) is retained only in the early Titanotheres (Osborn), in the remaining 

 families the number has increased as follows: Tapirus 18 + 5, Rhinoceros 

 19 or 20 + 3, Equus 18 + 5 — 19 + 6 (Flower, p. 83). Tail very early 

 becoming lighter than in the Condylarth-Creodont stem. 



Scapula rather high and narrow, round topped, prespinous fossa narrow 

 at top, anterior border above neck with sharp convexity. This type is readily 

 derived from the scapula of Phenacodus. It gives rise to various modifica- 

 tions, in the horses paralleling the Artiodactyl type, in the Titanotheres the 

 Proboscidean type. 



The humerus has progressed beyond that of any known Condylarth 

 in its cursorial adaptations. Even in the earliest Perissodactyls there was no 

 entepicondylar foramen and the entocondyle itself was reduced, the supina- 

 tor crest small, the capitellum for the radius transversely extended (more than 

 in Phenacodus) and the radio-ulna trochlea flatly convex. The head of the 

 radius was transversely expanded and there was a decreasing power of supi- 

 nation, the hand becoming more sharply pronated, even when extended 

 forward. 



Pelvis. The ilium, even in the Creodonts, had already lost its primitive 



