132 ]\lr. 0. Thomas on some 



teeth in each jaw, exactly as in the CebiJte, while the 

 invariable formula of the Callitrichidfe is PM. |, M, t 



In its general shape the sknll resembles that of a small 

 Saimirij the brain-case being higli and rounded and the 

 upper profile, from tip of nasals to occiput, evenly convex, 

 with no resemblance to the flattened forehead and prominent 

 brow-ridges of the marmosets. The orbits are not as slanting 

 as in Saimiri, more so than in Callkhrix. Malar part of 

 zygoma broadly expanded vertically. Anterior part of base 

 of skull deeply concave between the pterygoids, with a narrow 

 mesial septum, Pteiygoids shaped quite as in marmosets, 

 the ectopterygoid not so broadened as in most Cebidae. 

 Lower jaw with the well-marked chin and comparatively 

 vertical incisors of the Cebidse instead of the slanted symphysis 

 and incisors of the marmosets. Coronoid and condylar pro- 

 cesses nearer together than in either of the related forms. 



Molars, although less narrow, essentially of the triangular 

 type of those of marmosets, the internal ciiigulum well 

 developed, but with no distinct hypocone, the development of 

 this cusp being what causes the characteristic square form of 

 the molars of the Cebidse and other monkeys, including man. 

 In Sainiiri the cusp is less developed than in other Cebidae, 

 but is nevertheless always present. Lower incisors and 

 canines of normal relative proportions, not specialized as in 

 the genus CaUithrix. 



CalUmico thus proves to be almost exactly intermediate 

 between the otherwise well-defined families Cebida^ and 

 Callitrichidse, and it is quite a doubtful question as to which 

 of them it should be referred to. On the whole, in spite of 

 its marmoset-like claws and the structure of its molars, I am 

 inclined to place it with the Cebidte, of which it would form 

 a special subfamily. 



We should thus get the following arrangement of the 

 New-World monkeys: — 



A. Molars g. Skull rounded, forehead not flat- 

 tened Fam. Cehidce. 



a. Molars with hyi^ocones. Digits with 



shortened nails (often compressed) .... Subfam. Cebinm. 



b. Molars triangular, without hypocones. 



Digits with long claws Subfam. CallimiconincB. 



E. Molars |- Skull with flattened forehead. 

 Molars triangular, without hypocones. 

 Digits with long claws Fam. Callitrichida. 



It is to be noted that the difference in structure between 

 molars with hypocones and those without is not so abrupt as 



