no AMERICAN PERMIAN VERTEBRATES 



quite satisfied as to the value of this character. The relationships 

 of Varanosaurus with Theropleura Cope, as identified and figured 

 by Case, are very evident. But this genus includes much larger 

 forms with very noticeable differences in the structure of the 

 limbs. And it is also very apparent that the genus Ophiacodon 

 Marsh is a nearly related genus, clearly differentiated, however, 

 by the teeth and limb bones, as I identify them in the following 

 pages. These five genera, then, in the order of priority, Ophiacodon 

 Marsh, Theropleura Cope, Poliosaurus Case, Varanosaurus Broili, 

 and Poecilospondylus Case, so far as now known, are closely related 

 genera of the family Poliosauridae, distinguished by characters 

 given on a preceding page. I suspect also that Dimetrodon 

 navajoicus Case will be found eventually to belong in another 

 genus of the same group. 



The relationships of the family, using Varanosaurus as the type, 

 are in some respects very evident with the Clepsydropidae, in 

 others apparently divergent. Perhaps the chief relationships will 

 be found in the pelvis, and especially in the pubes. Both families 

 have a distinct pubo-ischiadic vacuity; both have the same peculiar 

 expansion of the pubes, with the outer border thickened and the 

 inner expanse horizontal or even convex above; both have the 

 same disproportionate size of these bones as compared with the 

 ischia. On the other hand, the Clepsydropidae have three sacral 

 vertebrae, while the Poliosauridae have but two. The vertebrae 

 are alike in the two groups, with the same rib attachments and 

 transverse processes. The feet, moreover, are quite similar; 

 and the spines, while of nearly uniform length, in Varanosaurus 

 are distinctly elevated and thin, not enlarged at their upper ends. 



Widely different, however, are the skulls in the two groups, in 

 the elongated form, almost isodont teeth, the incomplete temporal 

 arch, the sloping occiput, and the depressed form. 



Case believes that the Poliosauridae are the more generalized 

 of the Pelycosauria, and in some things they are, but not in all; 

 the incompleteness of the temporal arcade is certainly a significant 

 specialization. 



The resemblances between Varanosaurus and the so-called 

 Proganosauria are evident; whether they are really evidences 



