EDENTATA OF THE SANTA CRUZ BEDS. 



167 



(5) The vertebral column is completely known only in the Megalony- 

 chidce, partially in the Planopsidce and not at all in the Mylodontidce. In 

 the first named family the back-bone is very long and the vertebrae very 

 numerous ; the vertebral formula has been definitely determined only in 

 one genus, Hafialops, but the material is sufficient to prove that in several 

 other genera the formula must have been nearly or quite the same, and 

 there is no reason to suppose that there was any important difference in 



FIG. 19. 



Mandibles of Santa Cruz Gravigrada, x \. a, Hapalops indifferens ; b, H. rostratus ; c, 

 H. elongatus ; d, Eucholceops fronto ; e, Schismotherium splendens ? ; f, Prepotherium moyani ; g, 

 N ematherium sp. 



this respect throughout the family; this formula is: C. 7; Th. 21-22; L. 

 3-4; S. 5-6; Cd. 20. The epiphyses of the vertebras remain separate 

 till a comparatively very late period, after those of the long bones have 

 united with the shafts and many sutures of the skull have been obliterated. 

 The neck is of moderate length and no example is known in which the 

 number of cervical vertebrae is more or less than seven ; these cervicals 

 are relatively small and weak and are usually in striking contrast to the 

 large and massive lumbars of the same individual. 



The trunk, whenever the material is sufficient to permit a judgment 

 upon this point, is remarkably elongate and bears a highly suggestive 

 resemblance to that of Cholcepus. The centra, which in the anterior part 

 of the thorax are very small, increase in size posteriorly, becoming quite 

 large in the hinder part of the thoracic and the lumbar regions, and in 



