NORTH AMERICA AND THEIR VERTEBRATE FAUNA. 



139 



Dimetrodon navajovicus, but which certainly does not belong to that genus, 

 and has been called ArribasaurusJ^ 



Ophiacodon (fig. 25). — This creature is known from an almost complete 

 skeleton. The head was much more elevated in the facial region than in the 

 related Texas forms, approaching in that respect the genera Clepsydrops and 

 Dimetrodon from Texas. The peculiarly narrow skull, with the high and long 

 facial region, and eyes far up on the sides and near the posterior end, was 

 set at such an angle to the vertebral column that it was probably held rather 

 high. The teeth were sharp, compressed cones, with cutting edges, but nearly 

 isodont. The body was slender, with a tail as long as the body, and strong 



Fig. 25. — Restoration of Ophiacodon minis Marsh. About one-seventh natural size. 



limbs. The neural spines of the dorsal vertebras are somewhat elongated, 

 indicating the presence of a not very pronounced dorsal crest, as is suggested 

 in the restoration. The caudal vertebrae were without high spines or long 

 chevrons, indicating that the tail was cylindrical in form, and not of much 

 use in swimming. Probably the animal was more terrestrial than its relatives 

 in Texas. The total length of the single well-preserved skeleton is, as 

 mounted in the University of Chicago, about i meter; this includes the tail, 

 which is partly restored. 



Thcropleura (fig. 26). — Much like Varanosaurus in most characters, 

 Theropleura was larger and stronger than that genus, with broader and heavier 



:^ 





r^t » '*>. 





Fig. 26. — Restoration of Theropleura retroversa Cope. About one-tenth natural size. 



limb bones. The dentition shows an approach to that of Clepsydrops and 

 Dimetrodon in the beginning of enlarged incisors and maxillary tusks. The 

 animal was evidently able to attack and hold active and vigorous creatures, 

 which it encountered, in all probability, in the water. One suspects that when 



' Case and Williston, Carnegie Inst. Wash. Pub. i8i, p. 6, 1913. 



10 



