SINCLAIR: TYPOTHERIA OF THE SANTA CRUZ BEDS. 5 
is the presence in the first and second of a deep median cleft (PI. V, figs. 
13, 14) producing a fork-like effect, resembling the somewhat similar sub- 
division of the lower incisor crowns in the Hyracoidea. In all the Santa 
Cruz typotheres the enamel layer on the enlarged incisors tends to be 
confined to the anterior surface of the crown. The molars in all the 
genera are constructed on much the same plan, but only in Protypotherium 
are absolutely unworn teeth known, consisting essentially of a broadly 
concave ectoloph (e, text fig. 5, A) and a pair of internally convex cres- 
cents (ac, pc , text fig. 5, A ), of which the anterior horns are fused with the 
ectoloph, inclosing a reentrant. A crista-like ridge from the ectoloph 
(c, text fig. 5, A) is separated from the anterior crescent by a deep notch. 
A slight ridge (//, text fig. 5, A) blocks the shallow valley inclosed by 
the posterior crescent. As the tooth wears, the antero-external angle of 
the crown elongates and is channeled by a shallow groove, producing the 
ridges noted in the key to the genera. 
In the lower molars the convexity of the crescents is reversed, so that 
the reentrant fold is external (text figs. 1 , A ; 1 , D ; 6, A). A prominent lobe 
spanning the arc of the posterior crescent (pp, text fig. 6, A) is not pecu- 
liar to the teeth of the Typotheria alone, but is present also in Nesodon 
(text fig. 6, B), A strapotherium, Theosodon and other extinct ungulates 
from South America. In the last lower molar the development of the 
third lobe present in Interatherium is accomplished by the deepening of 
the shallow groove indicated in Protypotherium at the point marked 
pc in text fig. 6, A. 
As mentioned in the generic key, the premolars are sometimes molari- 
form and sometimes not, differing from the molars in the latter case in 
having the anterior crescentic lobe smaller than the posterior. 
Roots are developed only in the deciduous premolars, but as these have 
been observed only in Protypotherium and Interatherium , it is not alto- 
gether certain whether this character is of family or subordinal value. So 
far as can be ascertained, the crown-pattern seems to have been the same 
in the deciduous and permanent series, the milk-teeth resembling their 
successors. The order of replacement seems to have been the normal one. 
A thin layer of cement is usually observable on the molars and pre- 
molars of all the genera. 
3. Axial Skeleton. — The dorso-lumbar vertebral formula in Intera- 
therium is twenty-two, of which fifteen are dorsals. It was probably the 
