MESODONTA. 
135 
I. Last true molar with cusps in opposing pairs—Continued. 
AA. The anterior inner cusp undivided on all the molars. 
Last molar with heel; last premolar with inner cusp; four pre¬ 
molars . Hyopsodus. 
Last molar without heel; last premolar large, sectorial, without 
inner cusp . Apheliscus. 
Three or two premolars, last without inner cusp. Anaptomorpims. 
II. Last molar with a longitudinal series of alternating cusps, including a heel. 
Anterior inner tubercle of molars 1-2, bifid. Notharctus. 
Anterior inner tubercle simple. Opisthotomus. 
TOMITHERIUM, Cope. 
Third Account of New Vertebrata from the Bridger Eocene of Wyoming, p. 2, Aug. 7, 
1872; Proceedings American Philosophical Society for 1872 (published Jan., 
1873); On the Primitive Types of Mammalia Educabilia, 1873, May 6, p. 2; 
Annual Report U. S. Geol. Survey Terrs., 1872, p. 546 (1873). 
Dental formula of the inferior series: I. 2; C. 1; Pm. 4; M. 3. The 
last molar has an expanded heel. Tlie third premolar consists of a cone 
with posterior heel; the fourth premolar exhibits, besides its principal cone, 
an interior lateral one, and a large heel. The true molars support two 
anterior tubercles, of which the inner is represented by two distinct cusps 
in one or more of them, and the external is crescentoid in section. The 
posterior part of the crown is wide and concave, and is bordered at its 
posterior angles by an obsolete tubercle on the inner and an elevated angle 
on the outer side. In the T. rostratmi, the type of the genus, the middle 
incisors have transverse cutting-edges. 
This genus is allied to Adapis, Cuvier, of the French Eocene, but 
differs in the possession of but two incisors on each side; in Adapis, there 
are three, according to Filhol. From Notharctus and Opisthotomus, it differs 
in the structure of tlie last inferior molar, as exhibited in the analytical 
table. 
An account of the osteology of this genus, so far as indicated by my 
material, was given in the papers above referred to. It was shown that the 
hind limbs, especially the femur, are quite elongate, more so than the fore 
limbs, and that the proportions of both fore and hind limbs are slender. The 
head of the radius is subround, and its distal extremity a subequilateral 
triangle. The humerus is distally expanded, with large inner and outer 
epicondyles and an aiierial foramen. The tuberosities of its head are 
