I40 S. W. WILLISTON 



the families Clepsydropidae, Pariotichidae and Bolosauridae; his 

 Cotylosauria, the Diadectidae and Pareiasauridae ; the Proganosauria, 

 the Mesosauridae, Procolophonidae, Paleohateriidae, Proterosauridae, 

 and Rhynchosauridae (equivalent, it is seen, plus the Rhynchocephalia 

 and Choristodera, to Osbom's Diaptosauria). In 1891,' Cope defined 

 the Cotylosauria, now for the first time considered an order, as 

 including four families, the Diadectidae, Pareiasauridae, Parioti- 

 chidae, and Elginiidae. In the later publication he erected the 

 order Chelydosauria for the Otocoelidae proposed a few years before 

 for certain new reptiles from Texas, defining it as having the scapular 

 arch internal to the ribs, a dermal carapace, and the temporal roof 

 excavated posteriorly for the auricular meatus. Until this time 

 Pariotichus had been included among the Pelycosauria. In 1896,^ 

 however, he referred one species described as Pariotichus, P. hamatus, 

 to a distinct genus, Labidosaurus, which he provisionally placed 

 among the Pareiasauridae. 



In a few words, it is seen that Cope based the suborder Cotylo- 

 sauria upon the Diadectidae, and not until his later papers did he 

 unite any other American forms with it in the same group. In 1905, ^ 

 Case brought evidence to show that the essential characters assigned 

 to the Chelydosauria were also common to Diadectes and its allies, 

 and he has withdrawn the family from the Cotylosauria to include 

 it, with the Otocoelidae, in the Chelydosauria, leaving Pariotichus and 

 certain other less well-known forms as the sole American representa- 

 tives of the Cotylosauria. But this contravenes the basal rules of 

 nomenclature. The group originally was based exclusively upon the 

 Diadectidae, and, while we may add as many other families as we 

 choose, we may not substract the one upon which it was alone 

 based. The name Cotylosauria, of which Chelydosauria is purely 

 a synonym, must accompany the Diadectidae wherever the family 

 is placed. 



With the elimination of Chelydosauria we have three ordinal 

 terms which have been proposed for the primitive stegocrotaphous 

 reptiles: Cotylosauria Cope (suborder, 1880, order 1891); Pareia- 



1 Amer. Naturalist, p. 644; Syllabus of Lectures on the Vertebrates, p. 68. 



2 Proceedings of the American Philosophical Society, p. 136. 



3 Journal 0} Geology, No. 2, 1905, p. 126. 



