2G8 



The fossil in question consists of one-half of a vertebral body as repre- 

 sented in Figs. IG to 18, Plate XV. When resting upon the articular face, 

 it is not surprising that it should have been taken tor a '' petrified horse- 

 "hoof" by those not conversant with anatomy. 



The vertebral body in its entire condition would resemble in form those of 

 Megalosaurus, and in shape and other characters resembles those of Poicilo- 

 pleuron Bucklandi. This is an extinct reptile, from the Oolitic formation of 

 Caen, Normandy, described by M. Deslonchamps ; and remains apparently 

 of the same animal, from the Wealden formation of Tilgate, England, have 

 been noticed by Professor Owen. It has been viewed as a crocodilian, and 

 is estimated to have been about 25 feet in length. 



The Colorado fossil would indicate an animal approximating 40 feet in 

 length. 



One of the most remarkable characters of Poicilopleuron is the presence 

 of a large medullaiy cavity within the bodies of the vertebrae, as well as in 

 the long bones of the limbs. Among living animals I know of a similar con- 

 dition in the vertebrae of none except in the caudals of the ox. This curious 

 "feature is a striking one in the Colorado fossil, as represented in Fig. 18. 

 The lower two-thirds of the body appear occupied hy a large cavity, crossed 

 by a few osseous trabeculse. The cavity is bomided by a thick lateral and 

 inferior wall of compact substance, resembling that of the shaft of the long 

 bones of most mammals. The wall is about 2 lines thick, and thins away 

 at the upper part of the body where this is occupied by the ordinary spongy 

 substance. The latter extends into the abutments of the neural arch, and is 

 here more dense in character. The cavernons structure of the fossil is filled 

 with crystalline calcite. 



The estimated length of the vertebral body is about G inches. At the sides 

 and beneath it is much constricted or narrowed toward the middle. The 

 transverse section approaching the latter position is vertically ovoid, with the 

 lower and narrower end forming an acute angle. 



The articular end of the specimen, Fig. 16, is moderately depressed its 

 greater extent, most so above and becoming more superficial below. Its 

 upper border overhangs the deepest portion of the surface ; the lateral bor- 

 ders are obtusely rounded, and widen below in a strongly convex ledge, prob- 

 ably for the accommodation of a chevron bone. The bi'eadth of the articular 

 surface is nearly 4 inches ; Its vertical extent a little over that measurement. 



