NORTH AMERICA AND THEIR VERTEBRATE FAUNA. I33 



The large chevron bones and the elongate neural spines of the caudal 

 vertebrae show that the tail was expanded vertically, and was a most efficient 

 organ of propulsion. What effect the embolomerous condition of the verte- 

 brae may have had upon the strength of the vertebral column we can not say, 

 but certainly it can not be regarded as a weak structure. The edges of the 

 faces of the deeply biconcave vertebral disks meet upon a plane and would 

 permit little lateral motion of the column, if there were not a considerable 

 mass of inter- and intra- (between the central and intercentral disks) verte- 

 bral cartilage. If this were lacking, and certainly the disks seem very closely 

 set together in the few series known, the body would have been more or less 

 stiff, like that of an Icthyosaur or a modern dolphin ; but if, as seems more 

 natural, there was a considerable mass of cartilage, the body would have had 

 great flexibility. 



The animal probably moved with great swiftness, dashing upon fish and 

 smaller creatures, and seizing them in its powerful elongate jaws. A recently 

 discovered jaw indicates that the teeth were very small and recurved. 



The partially complete skeletons in the American Museum of Natural 

 History are from 30 to 40 centimeters in length, but the larger species must 

 have reached a length at least twice as great. 



In the recently discovered Brier Creek Bone Bed in Archer County, 

 Texas, great numbers of Cricotus vertebrae were found in association with 

 a fauna largely terrestrial, Dimetrodon, Edaphosaurus, Bolosaurus, Eryops, 

 and a few small amphibians, but no traces of sharks or fishes. If Cricotus 

 was largely aquatic in habit, as is indicated by its structure, this associa- 

 tion is difficult to explain. 



Chenoprosopus . — Though this form is known only from the incomplete 

 skull, the shape of the head is so similar to that of Cricotus that we may be 

 fairly certain that it resembled it in the rest of the body, and occupied the 

 same position in the waters over what is now New Mexico in Permo-Carbon- 

 iferous time as did Cricotus in the more eastern region. It is perhaps signifi- 

 cant that the skull of Chenoprosopus was found in a deposit of river sandstones 

 and that no vertebrae at all resembling those of Cricotus were found in the 

 immediately adjacent bone beds which were formed in the quiet waters of a 

 pool. Were Chenoprosopus, and perhaps Cricotus, members of a group which 

 normally lived in large bodies of water, but which more or less frequently 

 ascended the streams in search of prey, occasionally dying in places rather 

 outside of their normal habitat, and leaving their remains among those of a 

 fauna to which they did not normally belong? If so, they give us a glimpse of 

 an open-water fauna of which we have no other knowledge, unless it be the 

 radically different Stereosternum and Mesosaurus of the southern hemisphere. 



Mehl " has compared Chenoprosopus with Cacops and Archegosaurus, and 

 points out many resemblances. He is inclined to believe that the animal was 

 perhaps terrestrial, because of the lateral position of the orbits and the crush- 



" Carnegie Inst. Wash. Pub. No. i8i, p. ii, 1913. 



