70 Annals of the Carnegie Museum. 



pair, they articulate between the vertebral centra, thus giving rise to 

 demifacets to accommodate them. In Lasiopyga eight of these 

 articulate with the sternum by means of cartilaginous hsemapophyses 

 or costal ribs; in Callithrix only seven are so joined; the last three 

 in the first-named species are "floating ribs," while the costal ribs of 

 the ninth pair articulate with the lower margins of the costal ribs of 

 the eighth pair. The sternal ends of these costal ribs articulate 

 between the several pieces composing the sternum, which latter remain 

 separate bones apparently throughout life. A large, triangular 

 manubrium surmounts the series, and the xiphoid process at the distal 

 end is rather large and expanded, particularly in Callithrix. 



In Lasiopyga the three sacral vertebrae in the adult are firmly 

 fused together to form one single bone, the lateral processes of the 

 first being conspicuous'y thrown out to mold themselves on an 

 extensive iliac articulation of the pelvis upon either side. In Callithrix 

 only the two first sacrals thus coossify, and the third closely resembles 

 the first caudal vertebrae (PI. XIX, fig. 22). 



Lasiopyga callitrichus has the first three caudal vertebrae of a dis- 

 tinctive type. In some respects they resemble the last sacral verte- 

 bra, but all the processes are slenderer, more prominent and projecting. 

 Moreover, we find rudimentary chevron-bones here present, lapping 

 the centra, but not fusing with them. In Callithrix the three anterior 

 caudals more closely resemble the last sacral of that genus, and the 

 chevron-bones are small. The fourth caudal vertebra in L. calli- 

 trichus differs very markedly from the one which precedes it, as it is 

 more elongated, and its prezygapophyses are distinct, short, stout 

 processes directed forwards and outwards, and its postzygapophyses 

 are fused into one long, rather slender process, which springs from the 

 middle of the centrum to arch backwards. The lateral or transverse 

 apophyses are broad, outstanding, triangular lamellae and different 

 from those of any other vertebra anterior to it. There is a rudi- 

 mentary pair, which are stronger in the fifth caudal, after which they 

 become much reduced. In the fourth and fifth caudals, the chevron- 

 bones, completely ossified, are V-shaped in form, free, and articulate 

 with the centrum anteriorly and below, encroaching very slightly on the 

 vertebra next beyond. This encroachment is better marked on the 

 part of the chevron-bone of the fifth caudal, aftei which these elements 

 become double and distinct, and more and more rudimentary as we 

 follow the vertebrae distad until they finally disappear entirely on the 



