Page:The Osteology of the Reptiles.pdf/255

Rh of the various and diverse forms included in both orders, a better and more scientific division may be made on genealogical characters. 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 inconstant. 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 primitive 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 postfrontals often, the quadratojugals 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 Theromorpha 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 literis, with but few modifications.

Powerful reptiles from the size of a boar to that of a rhinoceros. Skull very massive, especially in the cranial region. Temporal opening bounded by the postorbital and squamosal, the jugal sometimes intervening below. Lacrimals and quadratojugals small, the interparietal and tabulars large. No dermal bones fused in midline. Parietal opening large, opening in a protuberance or boss. Teeth