Sinclair: typotheria of the santa cruz beds. 
49 
first and second notched by a deep internal groove. These teeth are 
proportionately broader than in Protypotherium and the groove does not 
bifurcate the summit of the crown as in the median lower incisors of that 
genus (Pis. V, fig. 22 ; VIII, fig. 25) but notches it as in I3 of Pvotypo- 
therium. The third incisor is cylindrical, with several shallow internal 
grooves. The canine and first premolar are simple cylindrical teeth, the 
former showing, in unworn specimens, a slight internal groove. The 
worn crowns of the second, third and fourth premolars and the first and 
second molars consist of a pair of triangular to oval lobes joined by a 
narrow isthmus like a figure 8 (PI. VIII, fig. 25). The third molar is 
rendered trilobate by the elongation of the posterior lobe and its constric- 
tion by a broad groove on the outer side. In slightly worn teeth it is 
seen that this lobate structure is developed from a pair of crescents having 
parts homologous with the elements of the crown pattern in the unworn 
lower molars of Protypotherium (PI. V, fig. 14 or, text fig. 6, A). A thin 
layer of cement is present on the lower premolars and molars. 
Milk Dentition. — The only difference between the deciduous premolars 
and their permanent successors is the presence of roots in the former (PI. 
VIII, figs. 22, 24). On the presence or absence of this character two 
genera have been established, Inter atherium with rooted premolars and 
Icochilus with these teeth hypsodont. These are merely the immature 
and adult stages of one and the same form. Interatherium has priority 
and must be retained as the proper designation for the genus. The order 
of tooth replacement is the normal one, but the milk teeth are retained in 
position until after the eruption of the third molar. It cannot at present 
be ascertained whether the first premolar has a deciduous predecessor, as 
it is a single-rooted tooth and has the same shape in specimens with 
rooted deciduous premolars as in those with hypsodont premolars. 
Sknll (PI. VIII, figs. 16-19). — The skull is short and broad, with 
heavy arches and prominent crests. The facial region is decidedly short, 
the orbit lying farther forward than in Pvotypotherinni. The premaxillae 
are heavy, with the ascending process short or wanting (PI. VIII, fig. 16). 
A prominent anterior nasal spine is present. On the palatal surface, the 
premaxillae are strongly arched antero-posteriorly and deeply excavated 
by the anterior palatine foramina, which extend posteriorly beyond the 
line of the premaxillo-maxillary suture. Along the line of this suture 
the rostrum is constricted vertically, producing a strong upward arching 
