I40 



THE PERMO-CARBONIFEROUS RED BEDS OF 



a Varanosaurus, or one of its kind, fled into the water to escape some prowling 

 Dimetrodon, it was in considerable danger of being devoured by its relative, 

 Theropleura. 



Sphenacodon. — From New Mexico came the scant remains of an animal 

 so like Dimetrodon in the structure of the skull that with the material at hand 

 it is impossible to distinguish the two genera by any cranial characters. The 

 dentition is in every respect the same, even to the enlarged incisors and 

 maxillary tusks, but as no vertebras of this animal have been found with 

 dorsal spines more than 20 to 30 centimeters in length, it is apparent that 

 Sphenacodon, while as active and fiercely carnivorous as Dimetrodon, lacked 

 the high dorsal fin. Perhaps it was even more to be dreaded by other ani- 

 mals, for the dorsal fin of Dimetrodon was most probably a serious hindrance 

 to rapid motion through the brush or high vegetation. It is very possible 

 that Sphenacodon, with dorsal spines just beginning to elongate, represents 

 the most active and efficient stage of the line which culminated in Dimetro- 

 don. The size of the animal is unknown, but one skull recovered is about 

 the size of that of a Dimetrodon of nearly 2 meters in length. 



Clepsydrops and Dimetrodo?i (figs. 27 and 28). — The line of the carnivo- 

 rous, raptorial reptiles reached its greatest development in these two forms. 



Fig. 27. — Restoration of Clepsydrops natalis Cope. About one-sixth natural size. 



In describing the amphibians and reptiles, constant reference has been made 

 to adaptations for defense, such as speed, agility, concealment, and armor. 

 Although the nearly universal rule of the fauna was to eat and be eaten, it is 

 probable that Clepsydrops and Dimetrodo7i were the most destructive forms 

 among the Permo- Carboniferous reptiles and probably the dominant cause 

 for the assumption of armor in their contemporaries. Clepsydrops was the 

 smaller and less speciaKzed of the two. The grasping-teeth were shorter and 

 weaker; the incisors of the lower jaw did not lock so strongly between the 

 premaxillary and maxillary tusks; the limbs were fairly long, but not so 

 strong or well articulated as in Dimetrodon; the dorsal fin was not so high. 

 The weaker articulations have led to the suggestions that the animal was 

 aquatic in habit, but this is, at least, doubtful. 



