l8 REVISION OF THE PELYCOSAURIA. 



The chief points of developmental evolution were : 



(i) The gradual change of the form of the skull from a low, elongate type 

 with a flat top, to a very high thin type with a narrow top. 



(2) The increase in the vertical curvature of the jaws with the development 



of a toothless (diastemalj notch between the premaxillary and maxillary. 



(3) The increase in size of the anterior incisors of both upper and lower jaws 



and the increase in size of the anterior maxillary teeth. The appear- 

 ance of serrations on the cutting edges of the teeth. 



(4) Increase in size of the external processes of the pterj-goid. 



(5) Increase in length of the neural spines until they became ver>- long and 



slender and finally developed transverse processes.* 



Family POLIOSAURIDAE Fam. Nov. 



Primitive Pelycosaiiria with low, flat, acuminate head, sometimes elongate. The 

 maxillary with straight tooth line. One or more teeth at the anterior end of the 

 premaxillary and dentary, and one or more teeth in the maxillar)- enlarged somewhat 

 above the others. Maxillary teeth not separated from the premaxillary- teeth by a 

 toothless interval. Vertebral spines low and the neural arch free from the centrum 

 through life in some {Poliosanrus). Abdominal scutes present. Long-bodied forms 

 with long tails ; probably aquatic. 



Dejiu ilk •€ characters :\ 



(i) Skull low and acuminate {Proierosanrmn). 



(2) Tooth line of maxillary straight or nearly so. Diastemal notch absent or 



poorly developed. 



(3) Neural spines short. 



(4) Sacrum with two vertebrse. 



Genus POLIOSAURUS Gen. Nov.J 



Proc. Am. Phil. Soc, vol. xvii, 1878, p. 519. Also Pal. Bull., 29. 



Type: Theropleiira uniformis Cope, No. 1148 Am. Mus. Nat. Hist., Cope, coll.; 

 from Texas. 



Original description (taken from T. relroz'ersa) : " Rhynchocephalian reptiles 

 with iree neural arches, and a capitular costal ailiculation on the centrum ; the inter- 

 centrum probably, and the hyposphene certainly, wanting. This genus is similar to 

 Lysorophus in its free neural arch, but there is no capitular costal articulation on the 

 known vertebrte of that genus." 



*Dr. Matthew has suggested that some readers may assume that this is meant to indicate stages of 

 phyletic evolution and not structural specialization of distinct genera. The Pelycosauria is as yet too inco- 

 herent a group for phyletic stages to be assumed; the points indicated are simply prominent stages in the 

 advance of the group to its overspecialization, indicated by different genera. 



tUnder this head in the descriptions of the families, genera, and species, the most prominent charac- 

 ters will be grouped under numbers that correspond in each family, genus, and species, so that comparisons 

 can be instituted directly. 



JThe type species, Theroplcura retroversa, was founded on a single vertebra which is indistinguish- 

 able from vertebrae of the same region in Diopeiis leptocephaliis, so the latter becomes a synonym. As a 

 second specimen, Thcropleura uniformis, is distinct and well characterized, it must receive a new name, 

 Poliosaurus (iroXios, hoary, venerable), and become the type of the new family. 



