EDENTATA OF THE SANTA CRUZ BEDS. 5 



A remarkable feature of the Santa Cruz edentates is their variability 

 within certain well-defined limits. As a rule, the genera may be readily 

 identified, but the species, especially of the Glyptodontia and Gravigrada, 

 present extraordinary difficulties to the systematist. This variability, how- 

 ever, confines itself to comparatively unimportant details, and the character- 

 istics of the three orders and of the families and genera within those orders 

 are already, for the most part, firmly established, though transitional forms 

 from species to species and, less commonly, from genus to genus abound. 

 i. The Santa Cruz edentates are relatively small animals and a few of 

 them are really minute. As compared with the ground-sloths and glypto- 

 donts of the Pampean, they are pygmies, but the armadillos have a greater 

 number of large species than exist at present, though none of them is 

 gigantic, or comparable to such a form as Macroeuphractus. 



2. Fully developed carapaces are found in all of the armadillos and 

 glyptodonts of the period, but, as yet, no dermal ossifications have been 

 found in connection with any of the ground-sloths. This is the less 

 surprising, because very little is known of the skeleton of the Santa 

 Cruz Mylodontidce, the only family in which these ossifications could be 

 expected to occur. 



3. The teeth are in all cases devoid of enamel, rootless and tubular, 

 though they may be lobate, examples of which occur in all three of the 

 orders. No trace of a milk-dentition has been observed. Premaxillary 

 teeth and the corresponding mandibular teeth have been definitely found 

 only in the armadillos, though rudimentary traces of such teeth are ap- 

 parent in some of the glyptodonts and they may also occur in a few of the 

 ground-sloths. 



4. The skull has few common features throughout the series, each order 

 having its own characteristic type of structure. The difference is largely 

 in the relative development of the cranial and facial regions, which varies 

 from the extremely elongate skull, with long, slender rostrum, of the arma- 

 dillos, to the short, broad, deep and almost cubical skull of the glypto- 

 donts. Sagittal and occipital crests are never very strongly marked, but 

 they are present in most genera of all three orders and there is no such 

 development of cranial air-sinuses as took place at a later period. In all 

 of the known genera from this formation except Peltephilus there is a 

 more or less prominent descending, suborbital process given off from the 

 zygomatic arch ; it may be formed by the jugal alone or by the jugal and 



