THE SUBCLASS SYNAPSIDA 237 



of the various and diverse forms included in both orders, a better 

 and more scientific division may be made on genealogical char- 

 acters. But such are not available at present. 



The characters, as a whole, of the Therapsida are primitive, but 

 less so than those of the Theromorpha, and they are increasingly in- 

 constant. The vertebrae are known to be notochordal only in the 

 Dromasauria and Dinocephalia, and the intercentra are seldom if 

 ever persistent throughout the column; there may be as many as 

 seven sacral vertebrae; the boundaries of the temporal opening are 

 less constant; in a few words, no characters seem to be more primi- 

 tive than in the Theromorpha. The interparietals, when present, are 

 fused into a single bone, which is rarely the case in the Theromorpha. 

 The supratemporals are always, the postf rentals often, the quadrato- 

 jugals usually, absent.^ The palate and teeth undergo many changes; 

 the pterygoids are less free, palatal teeth are less constant. The 

 cleithrum is seldom present and always small, etc. 



But to divide the various groups into orders seems not to solve 

 but rather to add to the difficulties. For that reason, perhaps it is 

 better at present to consider the whole group as one order, as Broom 

 has suggested, clearly differentiated from all others save the Thero- 

 morpha by the skull and pectoral girdle, and to treat its characters 

 under the chief divisions. Of course the distribution of some, perhaps 

 many, of the genera is more or less provisional, as must be the case in 

 any order of reptiles or other organisms until everything about them 

 is fully known, a result greatly to be wished, but never within the 

 limits of human endeavor. The classification adopted is that of 

 Broom and Watson in numerous publications and in Uteris, with but 

 few modifications. 



A. Suborder Dinocephalia 



Powerful reptiles from the size of a boar to that of a rhinoceros. 

 Skull very massive, especially in the cranial region. Temporal open- 

 ing bounded by the postorbital and squamosal, the jugal sometimes 

 intervening below. Lacrimals and quadratojugals small, the inter- 

 parietal and tabulars large. No dermal bones fused in midline. 

 Parietal opening large, opening in a protuberance or boss. Teeth 



* [The quadratojugal has recently been identified in anomodonts, gorgonopsians, 

 and cynodonts, by Watson. — Ed.] 



