EDENTATA OF THE SANTA CRUZ BEDS. 345 



MYLODONTID^E. 



In the Santa Cruz beds the representatives of this family are rare and 

 most of the specimens are more or less fragmentary, so that our knowl- 

 edge of these forms is still very incomplete. Except the skull, nothing is 

 positively known concerning the skeletal structure of the Santa Cruz 

 members of the family, but a few isolated vertebrae, limb and foot-bones 

 have been found, which may, with great probability, be referred to it. 



The family is most distinctively characterized by the teeth, which in 

 the upper jaw are triangular, and in the lower jaw lozenge-shaped, while 

 the last lower tooth is bilobate; the valleys are central and are usually 

 enclosed in a continuous wall of the harder dentine. The teeth of each 

 jaw are in continuous series and without diastema, and - 1 is always 

 implanted very far behind the anterior end of the maxillary. The forms 

 of the teeth are subject to great variation, not infrequently differing on 

 the two sides of the same jaw, and no two specimens are quite alike ; 

 slightly different modes of wear, which are doubtless individual, often 

 give a very different appearance to the teeth. 



In general appearance and proportions the skull is very similar to 

 that of the Santa Cruz Megalonychida, though with a number of constant 

 differences. The occipital crest is but slightly developed and the sagittal 

 crest apparently not at all ; the cranium is rounded and smooth, not flat- 

 ened on top, as it is in Mylodon, nor having a narrow sagittal area, such 

 as occurs in Scelidotherium. The parietals seem to have no sinuses and 

 those of the frontals are very small ; owing to the lower occiput and 

 shallower facial region, the upper contour of the skull has a much more 

 decided antero-posterior curvature than in either of the Pleistocene 

 genera. The lachrymal has the same prominent, mammillary shape and 

 funnel-like foramen as in the other two families. The face is elongate 

 and very slender, and the rostrum may have parallel sides, or it may 

 broaden anteriorly, though never so strongly as in Mylodon. 



The mandible differs in a number of details from that of the other two 

 families ; the horizontal ramus has a nearly straight ventral border ; the 

 predental beak is long and deeply concave and has nearly parallel sides, 

 with rounded or bluntly pointed anterior end. The condyle is placed 

 upon a shorter neck than in the contemporary Megalony chides, but is 

 raised higher above the level of the teeth, making the sigmoid notch 



