EDENTATA OF THE SANTA CRUZ BEDS. 279 



In the entire absence of the skeleton, it is impossible to speak with 

 confidence regarding the relationships of this genus, but, so far as can be 

 judged from the dentition and skull, there is no apparent reason why it 

 may not have been the direct ancestor of Megalonyx ; on the other hand, 

 it was clearly derived from the same stock as Hapalops. 



MEGALONYCHOTHERIUM ATAVUS sp. nov. 



(Plate XLVI.) 



This species, the type of which (No. 15,593) was found by Mr. Peterson 

 on the coast of Patagonia, about five miles south of Coy Inlet, is of 

 moderate size, about equal in this respect to Hapalops brachycephahts. 



The dentition has been sufficiently described in the generic diagnosis 

 and it only remains to notice that the two series of upper teeth are nearly 

 parallel and to give the dimensions. 



MEASUREMENTS. 



Upper dentition, length 06 1 --, length 007 



" 1--5- 033 " width 012 



1, length (i. e., antero-posterior diame- A, length 0085 



ter) 01 1 " width oil 



" width (i. e., transverse diameter) . . . .007 -, length 007 



^, length 0085 " width 008 



" width on 



As the most significant fact concerning this animal is its probable ances- 

 tral relationship to Megalonyx, it will be well to make a detailed compari- 

 son of the skull with that of M. jeffersoni. The occiput of the Santa 

 Cruz species, though of similar semicircular form, is relatively somewhat 

 wider and lower, and the foramen magnum is larger, though of the same 

 almost circular outline ; the median vertical ridge on the occiput is much 

 wider, more rounded and less prominent, and the occipital crest is more 

 distinctly defined and much narrower, not forming such a broad, rugose 

 surface ; the condyles are missing and the paroccipital processes, though 

 inconspicuous, are larger than in the Pleistocene' species. All of the ele- 

 ments forming the cranio-facial axis are indistinguishably fused together ; 

 this axis is relatively longer and narrower than in Megalonyx ; the basi- 

 occipital is similar in the two species, with convex median portion and on 

 each side a fossa into which open the condylar foramen and foramen 

 lacerum posterius. The mastoid portion of the periotic is quite exten- 



