82 



REVISION OF THE PEIvYCOSAURIA. 



The maxillary has a thickened inferior edge accommodating the sockets of the 

 teeth and a thin superior portion as in Dimetrodon. The thin upper portion is not 

 so high, nor is the thickened lower edge so wide or deep, for the teeth were not so 

 large and the sockets are correspondingly smaller. This is especially true in the 

 region of the enlarged maxillary, which has a socket not greatly larger than the rest. 

 The alveolar edge is straight. Anterior to the enlarged maxillar>- the thickened 

 margin plays out as the teeth become smaller. There are four small teeth anterior 

 to the enlarged maxillarj- and posterior to it nineteen can be counted, but a small 

 space is covered by a fragment of the lower jaw ; allowing five teeth to this space 

 there were twenty-four teeth posterior to the large maxillary. There are two large 

 maxillary teeth on the right side and a single one on the left. It is probable that this 

 genus had developed the same habit as the members of the Clepsydropidce, of alternate 

 functional teeth; at least it had made the preliminary step in the process by the 

 development of two canines equal in size and abruptly larger than the others. There 

 is a distinct upward curvature of the jaw anterior to the large maxillary, indicating 



Fig. 2 1 . — Restoration of skull of Theropleura relrooersa. About '4 nat. size. 



the first step in the development of the diastemal notch, but it is very slight and 

 there is no interruption of the teeth, although they are smaller. The alveolar edge of 

 the maxillar>' is straight. 



The nasals are represented by fragments only. 



The fyontals are represented by fragments and by a cast of the lower surface so 

 that nearly the perfect form can be made out. The cast shows the bones of the two 

 sides in nearly the correct position with relation to one another. They formed a flat 

 shield-shaped area 146 mm. in length and 104 mm. in width. 



The prefrontal is represented b)' a fragment onl)', but shows the same pit just 

 anterior to the orbit caused by the overarching of the roof of the skull that occurs in 

 Dimetrodon and ]'aranosaurjis. 



There is no trace of the lachrymal, jugal, postfrontal, postorbital, or parietal. 

 The outlines of the orbit are unknown. 



The base of the skull is lost, so there is no trace of the paroccipital, exocdpital, 

 basioccipital, or supraoccipital. 



The quadrate is a thin vertical bone with paired condyles for the lower jaw. 

 The condyles are low and far apart ; they limit the jaw to a strictly vertical motion, 

 but could not have held it rigidly as in Dimetrodon. The upper, thin portion is bent 

 so that it is convex outwardly. The upper side of the inner condyle is flattened, 

 forming a shelf on which rest the quadrato-jugal (?) as in Dimetrodon, but there is no 

 trace oi s. foramen quadratitm. The pterygoid articulates with the inner side of the 

 inner condyle as in Dimetrodon (plate 3, figs, i and 2). 



