Il6 REVISION OF THE PELYCOSAURIA. 



by the abundant occurrence of tlie bones as water-worn fragments in beds 

 of limited extent mixed with pebbles and sand ; indicating that the bones 

 have been transported a considerable distance by strong currents, perhaps 

 of rivers, and laid down in eddies or on river bars or deltas. The occurrence 

 of the Illinois material in what seems pretty certainly a Permian river bar 

 in Carboniferous rocks would support this idea. Only very rarely do the 

 bones of this genus occur in the fine clays deposited in the deeper water. 

 In such cases the skeletons are much more nearly complete, indicating 

 that the body of the animal floated and perhaps journeyed far to its final 

 resting place. 



In external form the animal must have been sufficiently bizarre, but 

 not more so than the living Phtynosoma or the Basiliscus of the West 

 Indies. The latter has a fringe of spines on the back which, though dermal 

 in origin, are nearly as high in proportion to the body as those of Dimetro- 

 don and originate and terminate almost as abruptly. As mentioned in the 

 descriptions, all the high-spined forms of both subfamilies have, just above 

 the base of the spine, a region of rugose lines of swellings and above this a 

 sudden change in the form of the spine. It is altogether probable that the 

 rugosity marked the termination of the muscular attachments, and that the 

 upper portion of the spine was covered by a coating of skin which, though 

 probably tough, was relatively thin. The outlines of the spines could very 

 probably be easily traced through the thin covering. For the rest the body 

 was probably not unlike that of a smooth-bodied, finely scaled lizard with a 

 relatively large head, short body and semierect posture. 



Haunting the banks of the streams or the upland, it probably lay in 

 wait and then made sudden, short, scuttling dashes upon its relatively slow- 

 moving prey, the Cotylosaurians, armored Chelydosaurians, and Amphib- 

 ians. That it attacked its own kind there is little doubt ; repeatedly the 

 spines are found with great exostoses, showing that they have been broken 

 and healed during life, evidences of sanguinary battles, perhaps in contests 

 for captured prey or for mates, or perhaps with the single notion of a can- 

 nibalistic meal. 



Dlmetrodon glgas Cope (plates 17-21). 



Characteristic specimens: No. 4006 Am. Mus.: The type. No. 1002 University of 

 Chicago: A large part of the vertebral column, the nearly perfect skull, the imperfect 

 vertebral column consisting of atlas, axis, third cervical, and another cervical, probably 

 the fourth, eight connected dorsals and ten others, posterior dorsals and lumbars, two 

 sacrals, eighteen caudals, and the femur. No. 4034 Am. Mus.: Fragments containing 

 a perfect neural arch of atlas. 



No. 1002 University of Chicago: The skull of Dimetrodon gigas has been 

 largely described in the characterization of the genus, but certain features distinguish 

 it from the related species. The simple diastemal notch of Dimetrodon incisivus 

 becomes a sharp offset, for the premaxillary is greatly shortened, and its tooth line 



