ANTHONY, FOSSIL MAMMALS FROM PORTO RICO 199 



generalized members known among the Megalonychidse and it is among 

 them that the nearest affinities to Acratocnus are found. The calcanenm 

 of Acratocnus resembles very much that of Hapalops, and the triangular 

 canine, characteristic of this new genus, is approximated rather closely in 

 some species of Hapalops. Acratocnus seems to have no very apparent 

 affinities with Megalocnus from Cuba, the former being at one extreme of 

 the series, the lighter limbed end, the latter being at the massive limbed 

 end of the series. 



To summarize, Acratocnus is a form widely differing from other ground 

 sloths in such a significant collection of characters as slender limb bones 

 (relatively speaking), deep anteorbital fossa, triagonal canines, large 

 molars, short diastema between c and pm4, no pronounced median, sym- 

 physial tongue, low-spined dorsal vertebrae and depressed, expanded 

 caudal vertebrae that its distinctness is readily apparent. Such sugges- 

 tive structures as the wide caudal and the low dorsal vertebrae, the facets 

 on the calcaneum calling for a long-necked astragulus, and the limb 

 bones, relatively long for their width, call for a consideration of the tree 

 sloths, the Bradypodidse, in this connection. It is not the purpose of this 

 paper to go deeph^ into such a question and the material might well be 

 deemed inadequate for such conclusions; but it appears well within the 

 limits of possibility that some such form as Acratocnus may be used 

 eventually to throw the two families, the Bradypodidse and the Megalony- 

 chidse into one family. Certainly the Porto Rico sloth was not so re- 

 stricted in its habitat as most of the members of the Megalonychidse and 

 if not in part, at least, arboreal, might readily become so. 



Regarding the age of Acratocnus, the indications point to its being a 

 contemporary of the Pleistocene or late Pleistocene period, quite certainly 

 not of a much earlier time. 



New GE^'us of Hystricomoeph 



The large hystricomorph rodent represented by the nearly complete 

 skull and the skeletal portions of several individuals proves upon exami- 

 nation and comparison with considerable material to be worthy of a new 

 genus and the following name is therefore proposed for it : 



Elasmodontomys obliquus ' gen. et sp. nov. 



Type, No. 14171, Dept. ^'ert. Pal., from the Cueva de la Ceiba, near Utuaclo, 

 Porto Rico, 1915 ; collector. Dr. Franz Boas. The type is a skull having all 



'"Elasmodontomys: iXaapLos, a thin plate; odwv = odovs, tooth; /j.vs, mouse — referring 

 to the thin plates of enamel that are found in the molars ; and oMiquus = oblique, these 

 plates being not at right angles to the tooth row, as might be expected, but decidedly 

 oblique to it. 



