636 Troxell- — Vertebrate Fossils of Rock Creek , Texas. 



Megalonychidm. 



Mylodon harlani. — The duplication of parts indicates the 

 presence of at least three individuals of the edentate Mylodon* 

 in quarry number one. Only broken parts of the skulls were 

 found and a few isolated teeth, but there seems to be almost a 

 complete specimen represented by the vertebrae and the limbs. 

 The huge pelvis, measuring about three and one-half feet across, 

 was, strangely enough, preserved in a sand bed, where other 

 bones were considerably worn by water action. 



Though a leaf-feeder, Mylodon is native to a semi-arid cli- 

 mate. A late South American immigrant, its presence in the 

 quarry cannot be later than the earliest Pleistocene. 



Elephantidm. 



Elephas fcolumbi. — A tooth, the proximal end of a femur, 

 and the middle portion of a tusk were found. These parts 

 are small, probably not more than two-thirds the size of a full- 

 grown elephant. The lamellae of the molar as well as the 

 smaller size indicate the Columbian or a young Imperial 

 mammoth. 



Elephas columbi does not appear until the early Pleisto- 

 cene ; if the identification of the species be correct, and it be 

 taken in connection with Platygonus and Mylodon, it indicates 

 that the layers of quarry number one are no earlier nor later 

 than the Equus beds. 



Summary. 



The material collected by the Yale Expedition of 1912 at 

 Rock Creek, Texas, included thirteen different species, 

 representing seven families. The living types to which they 

 are most nearly allied are the camel, dog, horse, elephant, sloth, 

 peccary, and turtle. 



Very satisfactory results were obtainable, especially in the 

 study of fragmentary material, by the use of " ratios." This 

 device, little used heretofore, not only served to associate the 

 parts of an individual according to size, but also pointed out 

 differences in proportion which were indicative of the varia- 

 tions between the species. 



An additional specimen of Equus scotti Gidley was found, 

 which is now mounted in the Peabody Museum. It seems 

 quite probable that E. scotti is a true horse and is not allied to 

 the ass or quagga, as was formerly supposed. 



Equus (Asinus) calohalus, n. sp., the holotype of which is 

 among the Rock Creek material, is distinguished by the 

 unusually long and slender limb bones. Of the modern types, 

 it is nearest to the ass, which it resembles very much. 



* Described by Professor Lull in this Journal for April, 1915. 



