144 



VERTEBRATE SKELETON 



is present. The Pelycosaurs (fig. 153) have both fossa — are diapsids. In all 

 the number of cranial bones is smaller than in Stegocephals. All four occipitals 

 are present, and usually the condyle is solely basioccipital, although occasionally 

 exoccipitals contribute, and in Cynognathus, recession of the basioccipital leaves 

 a pair of exoccipital condyles. 



Both supratemporal and tabulare are common in the roof and rarely there 

 is a preparietal in front of the parietals. Parasphenoid is lacking' and the 

 pterygoids usually meet in the middle line behind the vomer. An epipterygoid 

 is sometimes present and the quadrate is held firmly by the squamosal. The 

 number of bones in the lower jaw is less than in many living species; a goniale 

 being rare, while coronoid and surangulare are not common. The teeth are 



Fig. 153. — Skull of Dimeirodon, dorsal (Baur and Case), ventral (Broom), bo, 

 basioccipital; bs, basisphenoid; /, frontal; /, lacrimal; ?nx, maxilla; w, nasal; p, parietal; 

 pi, palatine; pm, prema.xilla; po, postorbital; pof. postfrontal; prf, prefrontal; pt, pter- 

 ygoid; q, quadrate; qj, quadratojugal; so, supraoccipital; sq, squamosal; 5/, supratem- 

 poral; V, vomer; x, bone called tympanic by Broom. 



thecodont (in distinct sockets) and in Theriodonts are difi'erentiated as incisors, 

 canines and molars, or as in many Anomodonts there is but a pair of large upper 

 incisors, or teeth may be entirely lacking. 



The PLACODONTIAhave both Theromorph (Synapsidan) and Sauroptery- 

 gian relations. The temporal fossa; are large, the orbits and nares lateral. The 

 exoccipitals exclude the supraoccipital from the foramen magnum. The small 

 squamosal is united to the quadrate, and a bone, interpreted as a quadratojugal, 

 extends down on the lateral side of the quadrate. Pterygoid and palatines, 

 meeting in the middle Hne, are fused to support the large pavement teeth which 

 also occur on the maxillae. An os transversum is present and the choanas are far 

 anterior. The lower jaw has an enormous coronoid, correlated with the crush- 

 ing teeth of the jaws. 



' Pantyliis is said to have a small parasphenoid in front of the basisphenoid, but the 

 pertinence of this and some other genera (c.g.,Scyiiionria, figure 152, A) to the Cotylosaurs 

 is questionable; they appear more like Stegocephals. 



