PATAGONIAN EXPEDITIONS! PALAEONTOLOGY. 



the metatarsals are far heavier, but retain their relative proportions. In 

 Analcimorphus and Pelecyodon, on the other hand, the difference in 

 length is very much less marked ; II and III are, it is true, much stouter 

 than IV and V, but they are not strikingly shorter, and V has no such 

 development of the proximal process as in Hapalops. 



, b 



FIG. 28. 



Distal end of right calcaneum, nat. size, a, Hapalops elongatus ; b, Prepotherium potens. 

 as, external astragalar facet ; as internal astragalar, or sustentacular facet ; cb, cuboid facet. 



The phalanges, which are all free, resemble those of the manus, but the 

 unguals are considerably smaller. 



The systematic arrangement of the Santa Cruz Gravigrada offers an 

 exceedingly puzzling and difficult problem. At that period the entire 

 group would appear to have been in a state of flux, as though the types 

 had not yet been definitely fixed, and I know of no series of fossil mam- 

 mals which so strongly suggests indefinite variations, from which a com- 

 paratively small number were chosen and fixed by natural selection. Not 

 only is there a great number of types, but these types are also, within 

 comparatively narrow limits, singularly variable and inconstant, so that it 

 is exceptional to find two individuals which are clearly and unequivocally 

 referable to the same species, and every imaginable transition may be 

 found. To depend upon the usual criteria, would result in the formation 

 of a different species for almost every specimen and the number of species, 

 already very large, would be greatly increased by the Princeton and New 

 York collections, in which few individuals agree at all closely with those 

 in the collections of Dr. Ameghino and the La Plata Museum. Differ- 

 ences of age may perhaps account for some of these variations and others 

 may possibly be due to sex, for there are, in a few instances, indications 



