NORTH AMERICA AND THEIR VERTEBRATE FAUNA. 107 



Upper portion of the head, show that such forms as Eryops, Acheloma, Trema- 

 tops, Trimerorhachis, etc., commonly lay nearly submerged in the water, as 

 do the alligator and the crocodile, while in wait for their prey, but there is 

 no suggestion in the skeleton of these creatures that they were powerful or 

 even active swimmers. The short and stubby feet, the broad limbs, the short 

 tails and plump bodies are those of sluggish creatures. Adaptation to an 

 active aquatic life is shown in the Amphibia by an elongation of the tarsals, 

 carpals, and to some extent of the phalanges. 



Cricotus was undoubtedly the most powerful swimmer yet discovered in 

 the fauna. The elongated body, with its long and slender tail; the strong 

 chevron bones and long, spinous process of the posterior caudals; the elon- 

 gated form of the skull; all these belong to an active, swift-swimming, 

 predaceous animal. 



The DissorhophidcB and AspidosauridcB form a group of amphibians which 

 were probably far more terrestrial in habits than those mentioned above. 

 The eyes were larger and more lateral in position. There is a large otic open- 

 ing, probably covered in life by a tympanic membrane. The limbs are heavier 

 and the feet shorter. The development of a dorsal armor would increase their 

 weight, and though this would not compel a terrestrial life, as is amply shown 

 by the presence of armor in Teleosaurus and the marine turtles, it is connected 

 with other structures, already mentioned, which suggest habits similar to 

 those of the land salamanders. 



Among the reptiles the adaptations are more varied than among the 

 amphibians, but not more susceptible of clear interpretation. In the Polio- 

 sauridcE {Poliosaurus, Theropleura, Ophiacodon, Varanosaurus, Varanoops, 

 and PcBcilospondylus) the body was elongate, with a moderately long tail; 

 the skulls somewhat lengthened, with lateral orbits, but in none do the feet 

 show aquatic adaptations, nor is there any lengthening of the chevron bones 

 or the spines of the caudal vertebrae which would indicate a swimming tail. 

 The bones of the feet, carpus, and tarsus, in the animals in which they are 

 known, are all well formed, with sharp articular surfaces showing a strength 

 only to be expected in a terrestrial or semiterrestrial animal, in which the 

 body was more or less habitually raised from the ground and supported upon 

 the limbs in running or walking. In only one genus, Varanosaurus, is the 

 complete foot known ; in this the strong phalanges terminated in well-developed 

 claws. In the other genera the feet are only partly known, but in the beds 

 where they occur, and in associations with the skeletons, are found an abun- 

 dance of small, sharp, well -formed claws, which must have belonged to them; 

 such are the claws of terrestrial animals. The members of this family prob- 

 ably dwelt upon the banks of streams and other bodies of water, not uncom- 

 monly taking to the water in pursuit of prey or for protection. 



In the family Clepsydropidce (Sphenacodontidce) , Clepsydrops and Dime- 

 trodon were relatively short-bodied, with tails of moderate length, and strong, 

 fairly long limbs, with well-knit carpus and tarsus. The digits carried strong, 



8 



