Il8 PATAGONIAN EXPEDITIONS: PALAEONTOLOGY. 



bricating rings, each of two rows of plates ; the number of these rings is 

 not definitely known, but was probably five or six. The plates have an 

 ornamentation similar to that of the carapace, except that the peripheral 

 figures are, at least in most of the rings, confined to the anterior portion 

 of each scute. In the hinder part of the tail the rings are but slightly 

 movable, and of these there appear to be four, each of two rows of plates ; 

 the scutes have no sculpture, but are finely punctate and retain a number 

 of conspicuous piliferous pits. A peculiarity of this genus is the abrupt 

 termination of the tail-sheath, the tube being closed by a small, irregular 

 plate. 



The material for the study of the endo-skeleton is unusually complete 

 in this genus. Besides numerous skulls and parts of the limbs, it com- 

 prises the beautiful specimen in the La Plata Museum, which was described 

 by Mercerat ('91) and figured by Lydekker ('94, PI. XXXII); a large 

 part of the skeleton of P. minor found with the fine carapace shown in 

 Plate XVII, and belonging to the American Museum of Natural History ; 

 finally the greater part of the skeleton of P. aitstralis associated with the 

 carapace figured in Plate XVIII, and now in the Princeton Museum. 



Dentition (Plate XXIV, figs. 1-6, 14). The dental formula is f, but in 

 the premaxillary and corresponding part of the mandible are to be 

 observed two or three small pits, indicating the presence of additional 

 vestigial teeth, which were shed very early. This fact, which was first 

 pointed out by Ameghino (87*, 24), has been denied by Lydekker ('94, 

 47), but it occurs in too many specimens to be without significance. Of 

 the functional teeth in the upper jaw, - is the smallest, but it varies much 

 in size and in its position with reference to the maxillo-premaxillary suture ; 

 in form it is either cylindrical or D-shaped, with major axis transverse and 

 convexity turned forward. The next tooth, -, is always larger than A , but 

 even more variable in size ; it has an elongate elliptical shape and a feebly 

 marked vertical groove on the inner side ; ^ is still larger and is made ex- 

 ternally bilobate by a broad sulcus and on the inner side feebly trilobate 

 by two shallow vertical grooves ; - is similar, but larger and with much 

 more distinct lobation ; A is trilobate both internally and externally, but 

 with much larger inner lobes ; in many individuals a distinct vertical 

 groove on the anterior face demarcates the antero-external from the antero- 

 internal lobe and the postero-external lobe is invariably divided into two 

 pillars by a shallow vertical groove. The remaining teeth, & , ^ *, resem- 



