EDENTATA OF THE SANTA CRUZ BEDS. l8l 



widely separated posterior branches, of which the external one is broader 

 and flatter. The edentulous beak of the mandible varies in correspond- 

 ence with the length of the premaxillaries, but always contracts forward 

 gradually and is not deeply constricted in front of T ; the postero-external 

 opening of the dental canal varies much in position ; it is usually in the 

 ascending ramus, but in several species is in the horizontal ramus at the 

 base of the coronoid. The latter position has been used by Ameghino 

 as the distinguishing mark of Pseud hapalops, but transitions are not 

 wanting, as is shown in the mandible figured in Plate XL, fig. 4, and in 

 text-figure \$, p. 167. 



The vertebral formula is: C. 7 ; Th. 21-22; L. 3-4; S. 5-6; Cd. 20. 

 The neural spines of the trunk-vertebrae are short, straight and, especially 

 in the hinder portion of the trunk, are plate-like and broad antero- 

 posteriorly. 



The manus does not differ very markedly from that of other contempo- 

 rary members of the family in which the fore-foot is known, its principal 

 characteristic being the elongation of metacarpals IV and V. The femur 

 is broad and much compressed antero-posteriorly and the pes is quite 

 characteristic. Metatarsals IV and V are much longer and more slen- 

 der than II and III and there is an extremely prominent process on the 

 fibular border of the proximal end of V. 



The species of Hapalops are very numerous, but the problem concern- 

 ing the number and limits of these species cannot be solved with the 

 available material. What has already been said as to the extreme diffi- 

 culty of the taxonomic arrangement of the Santa Cruz Gravigrada applies 

 with particular force to this genus, in which variability is so remarkable. 

 The arrangement adopted in the following pages is to be regarded as 

 merely tentative and subject to revision as more complete specimens are 

 obtained. 



Relationships. That Hapalops is nearly related to both Nothrothe- 

 rium and Megalonyx is abundantly clear from a cursory examina- 

 tion of the skeleton, all parts of which are very similar to those of 

 the two Pleistocene genera. In fact, there is every reason to believe 

 that Nothrotherium was the direct descendant of some species of this 

 genus. On the other hand it is probable that Megalonyx was derived, 

 not from any Santa Cruz species of Hapalops, but from a closely allied 

 contemporary genus, Megalonychotherium. 



