Marsh Collection, Peabody Museum. 



205 



and the last is the smallest, being considerably reduced. In the 

 crowns of the first and second, the trigonal ridges are distinct, 

 and there is a prominent intermediate cusp present. The pos~ 

 tero-internal cusp is well developed and has a position much 

 more internal to the antero-internal than is generally the case 

 in molars of the higher Primates. Because of its position this 

 cusp is more widely separated than usual from, and does not 

 develop a close connection with, the original trigon. There is 

 a strong cingulnm continued forward around the inner face of 

 the crown, from which a small cingular cuspule is formed 

 internal to the main antero-internal cusp. This is most dis- 



139 140 



ia i i i«- 



Figures 139, 140. — Upper and lower teeth of Chrysothrix sciurea ; crown 

 views ; twice natural size. 



tinct in the second molar, although a considerable rudiment of 

 it is seen in the h'rst. In the squirrel monkey, Chrysothrix 

 sciurea, figures 139, 140, and 141, the superior molars exhibit 

 practically the same structure as those of Cebus, the only 

 important difference between the two being that the posterior 

 intermediate cusp is not distinct in Chrysothrix. The cingu- 

 lum is continued around the inner face of the crown in both 

 the first and second molars in the same way as it is in Cebus / 

 but its development is greater in the first molar than in the 

 second, whereas in Cebus the cingulum and the anterior cingular 

 cusp are stronger in the second than in the first. 



This peculiarity in the structure of the molars is not found 

 in any other South American ape, nor, as far as I am aware, in 

 any other living species of Primate in any part of the world. 

 It is highly significant, therefore, that so unusual a modifica- 

 tion and one so entirely unique among the Primates should be 



