SAUROPTERYQIA. 159 



tigriceps). Ptychognathus (Lystrosaurus), from the same forma- 

 tion, and also from corresponding deposits in India, is smaller. 

 Toothless forms of skull from the Karoo Formation are named 

 Ondenodon, but may possibly be referable to the females of 

 Dicynodon. Closely related genera are represented by small 

 species in the supposed Trias of Elgin, Scotland, one of these 

 (Gordonia, fig. 99) much resembling Dicynodon in the form of 

 the skull, the other (Geikia) similarly suggestive of Ptycho- 

 gnathus, but the first with only diminutive tusks, the second 

 absolutely toothless. 



ORDER 2. SAUROPTERYGIA. 



Another primitive group of reptiles in which the investing 

 bones of the temporal region of the skull contract into a single 

 broad zygomatic arch, is represented by small amphibious 

 animals in the Trias and by larger truly aquatic animals 

 throughout the remainder of the Mesozoic period. Notwith- 

 standing their ultimate complete adaptation to life in the open 

 sea, these reptiles always retain their two pairs of limbs and 

 their lizard-like form ; they are thus named SAUROPTERYGIA 

 in contradistinction to the Ichthyopterygia, which have an 

 extremely shortened neck and are quite fish-like in shape. 



The skull in the Sauropterygia exhibits a pair of large 

 supratemporal vacuities and also a pineal foramen. The conical 

 teeth, forming a single series orr the margin of the jaws, are 

 placed in distinct sockets ; and there are no teeth on the palate, 

 except rarely in some of the Triassic genera. The posterior 

 nares always occupy their primitive position on either side of 

 the vomers, and are never covered by secondarily-developed 

 palatine plates; the relatively large pterygoids meet in the 

 middle line, leaving no interpterygoid vacuity in the earliest 

 genera, and only a small vacuity in the later genera. No 

 sclerotic plates have been observed. The number of cervical 

 vertebrae is always considerable, but is largest in certain genera 

 from the Upper Jurassic and Cretaceous ; while the tail in the 

 typically marine forms is invariably short. The ribs in the 

 cervical region are remarkable as articulating only with the 

 vertebral centrum, never even in part with its arch ; although 

 nearly all these are distinctly double-headed in the earliest 



