EDENTATA OF THE SANTA CRUZ BEDS. 347 



but in some specimens they are almost horizontally abraded ; * or A is the 

 largest of the series and - the smallest and least distinctly triangular. 

 Even more variable are the lower teeth ; T is somewhat elongate antero- 

 posteriorly, laterally compressed and" of either triangular, or oval section, 

 and usually with bevelled crown ; in several instances - has worn a deep, 

 step-like notch on the posterior side of T ; Y and are four-sided and 

 lozenge-shaped, with the long axis of the crown oblique to the line of the 

 jaw ; T is bilobate and very elongate antero-posteriorly ; the anterior lobe is 

 transverse, but somewhat oblique, while the posterior lobe has a fore-and- 

 aft direction, curving inward at the hinder end, and is convex externally 

 and, except in one specimen, concave internally. 



Aside from mandibles, the skull is known only from three specimens 

 in the Ameghino collection, one of which, the type of Ammotherium 

 acitleatum (Plate LXII, figs. 2, 2, 2<) is quite complete and has the 

 mandible, lacking the ascending ramus ; another, the type of Lymodon 

 perfectus (Plate LXII, figs, i, \a] is the right half of a skull, without 

 lower jaw ; the third, the type of N. longirostris, is very incomplete. The 

 same collection contains also two palates, with teeth, referred respectively 

 to Ammotherium declivum and A. profundattim, but these add little to 

 our knowledge of the skull structure in the genus. 



In proportions this skull does not differ at all radically from that of the 

 Santa Cruz Megalonyc hides, the most striking distinction being the length 

 and slenderness of the region in front of the brain-case. The occiput is 

 broad, quite low, and inclined slightly forward ; the condyles are widely 

 separated by a broad ventral notch and the paroccipital processes seem to 

 be obsolete ; the occipital crest is inconspicuous and the supraoccipital is 

 reflected well over upon the dorsal side of the cranium. The brain-case 

 is full and rounded, without sagittal crest or area, narrowing regularly 

 forward to the interorbital region ; the zygomatic arch is imperfectly 

 known, but seems not to have been unlike that of Hapalops. The post- 

 orbital constriction is very shallow and inconspicuous and hence the long 

 frontals have little of the hour-glass shape seen in the Megalonychidce, 

 and the sinuses produce no noticeable elevation of the forehead. In front 

 of the postorbital constriction the skull is remarkably slender and elongate, 

 one of the most characteristic differences from the two preceding families ; 

 the maxillaries are produced far in front of the first tooth and the pre- 

 orbital fossae are very shallow, forming no distinct constriction when the 



