SCOTT: TOXODONTIA OF THE SANTA CRUZ BEDS. 289 
have coalesced to form a smooth continuous outer wall or ectoloph, in 
which the indication of the constituent elements is but feebly shown or 
entirely absent. The two oblique, transverse crests, proto- and metalophs, 
are fused with the external wall and the internal cusps, except in some of 
the more ancient genera of the Entelonychia, in which the inner cusps are 
described as being more or less separate from the crest, and doubtless this 
was the primitive condition in all of the suborders. The external wall is 
continued well behind the posterior crest, enclosing with it a fossa, which 
varies in size and distinctness. In addition to these principal crests, a 
variable number of cristse or spurs project inward from the ectoloph, which 
are most prominent and important in the commoner Santa Cruz Toxo- 
donta, in which one of these cristas is especially large, persisting even in 
well worn teeth and giving to the valley its characteristic Y-shape, while 
in Stenotephanos , which is a rare and imperfectly known genus, m 1 and - 
have a simple undivided valley, a character which recurs in all the 
molars of the post-Santa-Cruzian genera Xotodon and Toxodontherimn. 
In Toxodon , on the contrary, this principal crista is very prominent and 
larger every way than in Nesodon and Adinotherium. The enamel is 
not continuous around the whole periphery of the crown, but covers the 
external side and forms vertical bands on the other sides. The Leontini- 
idae of the Deseado stage have a slit-like, undivided valley, agreeing in 
this respect with the Entelonychia, in which the spurs are short and are 
speedily removed by wear. In the Typotheria the valley is very shallow 
and the crests low, so that even in young adults the grinding surface is a 
smooth expanse of dentine with an enclosing border of enamel and, in 
some genera (e. g. Hegetotherium ), a thin coating of cement. On the 
inner face of the crown, however, the vertical groove which marks the sep- 
aration of the two internal cusps is persistent. 
Skull. — The unity of the order is nowhere better displayed than in the 
skull, especially in the remarkable structure of the auditory region, to which 
Roth (’03) was the first to call attention. The peculiarity consists in the 
relatively great size of an element which Roth regards as homologous 
with the mastoid in man, but not with the element which is so called in 
other ungulates and which he designates as the pvotuberancia petrosa. 
The occiput proper is very broad at the base, is sharply constricted above 
the foramen magnum and then widens again moderately to the lamb- 
doidal crest. The large area on each side which would be left vacant by 
