NORTH AMERICA AND THEIR VERTEBRATE FAUNA. 



141 



The skull was high and narrow, with a large facial region, and eyes far 

 back in the skull ; the body probably rather slender, and the tail long. The 

 extreme length was probably somewhere near a meter or a meter and a half. 

 The animal probably haunted the banks of pools and streams, or stole through 

 the vegetation, watching its opportunity to pounce upon some helpless or 

 unsuspecting prey. 



In Dimetrodon the characters exhibited in Clepsydrops are developed to the 

 extreme. The morphological stages of development up to this highly preda- 

 ceous reptile are probably represented by Theropleura, Sphenacodon, and 

 Clepsydrops. The most primitive of the active carnivores were isodont reptiles 



Fig. 28. — ^Restoration of Dimetrodon incisivus Cope. About one-fifteenth natural size of an average speci- 

 men. The bade of the animal in the picture seems to be a little too much curved, but is modeled after 

 the nearly complete specimen in the American Museum of Natural History. 



like Poliosaurus and Varanosaurus, but as the necessity for seizing and holding 

 an active, struggling prey became greater, the powerful incisor and maxillary 

 tusks were developed, and the cheek-teeth of the skull and jaws became re- 

 curved, with sharp, serrate edges. There is no animal known, recent or fossil, 

 in which a more efficient apparatus for such cruel business has been developed. 

 With the increasing strength and size of the genus Dimetrodon, the limb 

 bones and feet became stronger and better developed. The animal probably 

 abandoned the aquatic habits of its ancestors and ranged very widely over 

 the land. As is apparent from the foregoing descriptions, the edges of the 

 pools were probably the regions most densely inhabited by amphibians and 

 reptiles, and no doubt such places were favorite haunts of the Dimetrodon. 



