Marsh Collection, Peabody Museum. 137 



series and there is good evidence that the incisors were not 

 very procumbent in position. The two halves of the lower jaw 

 were not coossified, even in the most aged individuals. 



In the upper jaw, figure 131, the teeth occur, in many 

 examples, in association with those of the lower series. The 

 structure of the molars is distinctive, not only by reason of the 

 rather sharply quadrate outline of their crowns, but also because 

 of their relatively great transverse extension. The first and 

 second are subequal in size, and the third is considerably 



131 



Figure 131. — Superior molars and fourth premolar of Hemiacodon gracilis 

 Marsh ; crown view ; two and one-half times natural size. 



The representation of the forward extension of the internal cingulum of 

 the molars, as well as the size of the internal cingular cusp, is somewhat 

 exaggerated. 



reduced. The external cusps are moderately flattened upon their 

 outer side, especially the posterior one, and they are bordered 

 externally by a strong basal cingulum. The intermediates are 

 unusually well developed, and there is a large internal pyramidal 

 cusp. A small postero-internal cusp is developed from the cingu- 

 lum, which continues forward around the inner side of the crown, 

 and gives rise to a moderately strong subsidiary internal cusp. 

 The extent to which this cusp, as well as the forward extension 

 of the cingulum, is developed, however, appears to vary in the 

 different specimens ; in some the cusp is very distinct, while in 

 others it is scarcely visible. The fourth premolar has single 

 external and internal cusps. The third and fourth premolars 

 are implanted by three roots, two external and one internal. 

 The teeth anterior to these are unknown with certainty. The 

 enamel is finely rugose in both the upper and the lower teeth. 

 In the fragment of a maxillary here figured, the anterior 

 limits of the malar can be easily made out. It is thus shown 

 that it does not reach forward to the lachrymal, but leaves the 

 maxillary a considerable share in the anterior boundary of the 

 orbit, as in the monkeys. The rather small, single, infraorbital 

 foramen is situated above and opposite the posterior edge of 

 the third premolar, about in the same relative position as that 

 of the squirrel monkey. The maxillary gives further evidence 

 of proportionally large orbits, and if the superior dental for- 

 mula was the same as that for the lower jaw, the muzzle must 

 have been considerably abbreviated. The whole aspect is, in 



