Page:The Osteology of the Reptiles.pdf/264



, large temporal opening, bounded above by the parietal, below by the postorbital and squamosal. No dermosupraoccipitals, tabulars, or quadratojugals. Quadrate fixed. A parietal foramen. Neck elongated, the tail never long. Vertebrae platycoelous. Cervical ribs attached exclusively to the centrum, the dorsal ribs exclusively to the arch by a single head. A single, large coracoid on each side. Girdles stout. Pelvis with large pubo-ischiatic opening, or secondarily a thyroid foramen. No sternum. Parasternals stout.

There is still much doubt as to the derivation and genealogical relationships of this order of reptiles, chiefly because of the structure of the temporal region. The general characters of the skeleton are more or less modified by aquatic adaptations. The boundaries of the temporal region seem to be those of the upper opening of the diapsid reptiles; and there are many who believe that it really is the upper one, and that the order is nearest related to the Proganosauria. The opening, it is seen, is bounded quite like that of some members of the Therapsida, especially the Cynodontia; and these reptiles are confidently believed to have descended from theromorphous reptiles with a typical lower opening. The more general opinion is that the Sauropterygia are related to the anomodont-like reptiles. Some, however, would trace their descent directly from the Cotylosauria; others from the Diapsida, by the loss of the lower arch. The author believes that the first of these views is the correct one, but in the present uncertainty they may be left in an independent group.

Whatever has been their origin, we must await the discovery of their more terrestrial ancestors in the early Trias. The modifications of structure in adaptation to aquatic life are very pronounced, even in the Nothosauria. The order is clearly divisible into two chief groups, the Nothosauria and the Plesiosauria.