322 PATAGONIAN EXPEDITIONS : PAL/GONTOLOGY. 



Evidently the significance of these differences is that a beginning had been 

 made in the rotation of the foot, so as to bring the weight upon the fibular 



border. 



PLANOPS Ameghino. 



(Plates LVIII ; LIX, Figs, i-i 6 .) 



Planops Amegh. ; Enumeracion sistem., etc., 1887, p. 23. 

 Schismotherium Mercerat, in part; Rev. del Museo de La Plata, T. II, 1891, 

 p. 8. 



In the collection of the American Museum of Natural History is the 

 greater part of a skull of P. longirostratus and in the Princeton collection 

 is a nearly complete skull, lacking the mandible, of a new and much 

 larger species ; both skulls represent quite immature animals, as all the 

 sutures are still open. These specimens give us much welcome informa- 

 tion regarding a genus which has hitherto been so imperfectly known, 

 but of the skeleton nothing has yet been found. 



The dentition is characteristic and seems to be unusually constant ; - is 

 small, cylindrical and caniniform ; it is implanted very far behind the 

 anterior border of the palate and is followed by a considerable diastema. 

 The remaining teeth are molariform and of transversely oval or subrect- 

 angular shape ; a is the largest and - the smallest of the series, and the 

 latter has on the posterior face a median vertical groove, which gives the 

 grinding surface a reniform shape. All of the teeth are small in proportion 

 to the size of the animal and the two series are nearly parallel. 



The skull has neither sagittal crest nor temporal ridges ; the rostrum 

 is unusually long and has almost parallel sides and the preorbital fossse 

 are very shallow ; the palate is but slightly convex between the molari- 

 form teeth and less roughened than in the preceding genera, and ante- 

 riorly is flat or even somewhat concave ; the premaxillae were evidently 

 large, the facets for them upon the maxillaries being of uncommon size, 

 while the median notch for the spines is a characteristically deep and 

 narrow V. 



That Planops is very closely allied to Prepotherimn is clear ; indeed, 

 more extensive material may perhaps show that the two are identical. 

 On the other hand, the likeness to AnalcimorpJms is unmistakable and 

 the latter genus connects the present group with the Megalonycliidce. 

 The probable relationships between the Planopsidcc and the Megatheriidce 

 can be most conveniently discussed in connection with Prepotherinni. 



