Marsh Collection, Peabody Museum. 31 



somewhat concave. A sharp ridge descends from the apex of 

 the main cusp in front, curving gently inward to terminate at 

 the base of the crown in a small, though distinct, anterior cusp 

 developed from the cingulum. Posteriorly the principal cusp 

 is flattened in such a manner as to present a triangular' face 

 looking upward and backward. Upon the descending ridge 

 forming the inner border of this triangular area is a small, but 

 distinct, internal cusp, which stands internal and posterior to 

 the main cusp. The posterior face of the crown descends 

 steeply to an indistinct ledge at the base, which is the rudiment 

 of the heel. There are no additional cusps developed, however, 

 in this situation, and the heel may be said to be practically 

 rudimental or absent. It is a matter of importance to note the 

 relations of the cusps, since they serve to explain the struc- 

 ture of the succeeding molars. There is a slight cingulum 

 surrounding the base of the crown in front. 



The first and second molars are 

 of nearly equal size, and like those 

 of so many other primitive Primates 

 their crowns are composed of an 

 anterior, narrow, triangular portion 

 bearing three cusps — the trigon, 

 and a broader posterior basin-shaped 

 moiety — the heel. The trigon of the Figure 120.— Fragment of a 



first molar is most distinct, and the right lower jaw of Omomys Car- 

 .-. , -. , ten Leidy (type of Hemiacodon 



three subequal cusps are arranged nanus Marsh) . ins ideYiew; two 



111 the form of an equilateral tri- and one-half times natural size. 



angle. If the external cusp is taken 



as the apex, the base coincides almost exactly with the tooth 

 line. It results from this arrangement that the internal cusp 

 is situated posterior and internal to the external cusp, which 

 corresponds to, and is strictly homologous with, the main cusp 

 of the premolar in advance. The anterior cusp of the trigon 

 is well developed, of. a distinctly conical form, and projects 

 slightly forward in advance of the crown. The heel broadens 

 rapidly, and is composed of a large Y-shaped external, and 

 a smaller, more or less conical, internal cusp, inclosing a 

 depression or valley. This valley is completed behind by 

 two ridges passing inward and backward from the two cusps 

 of the heel. At the angular point where these two arms meet, 

 a slight swelling of the enamel occurs, which may be spoken 

 of as an additional cusp. 



The second molar differs but little from the first, the only 

 noticeable variation being that the anterior cusp of the trigon 

 is considerably smaller, less conical, and occupies a more pos- 

 terior position. The external and internal cusps of the trigon 

 are likewise more nearly opposite each other, or transverse to 

 the long axis of the jaw. 



