PROTOROSAURIA 135 



about twice the length of the dorsal vertebrae following them. 

 The cervical ribs are very slender bones, articulating by a single 

 head with the centrum only. In these and other characters, so 

 far as they are known, Araeoscelis seems to agree with Protorosaurus, 

 and both have very hollow bones. 



PLEUROSAURUS 



We may for the present be justified in maintaining the order 

 Protorosauria for those reptiles having a single, typically upper 

 temporal opening on each side, with a fixed quadrate, not includ- 

 ing the ichthyosaurs. It is not improbable, however, that when 

 more is known of the ancestors of the lizards, the whole group will 

 find its most natural place among the Squamata. This definition 

 will include a peculiar aquatic reptile that has been known for 

 many years, but which has been wrongly classed in the same family 

 as Sphenodon, on the purely gratuitous assumption that it has two 

 temporal openings on each side; we now know that it has but one. 

 This reptile, known scientifically as Pleurosaurus, was described 

 originally by H. von Meyer in 1843, but we are indebted to M. 

 Lortet for a more precise knowledge of the animal, and for the 

 figure (Fig. 63) which is here given of the skeleton. Not a few 

 excellent skeletons are preserved in the museums at Lyons and 

 Munich. The specimen here figured, as actually preserved, 

 measures about three feet in length; a part of the tail is missing, 

 which is known from other specimens to have been remarkably long. 



The figures show clearly some of the remarkable aquatic 

 adaptations of the animal, especially the short neck, the very long 

 and narrow body, and the extraordinarily long and flattened tail. 

 The head is elongate triangular in shape, resembling very much 

 that of the mosasaurs ; and the external nostrils are likewise situated 

 remotely from the end of the snout, as in the mosasaurs. The 

 extremity of the snout has a beak-like projection. The teeth are 

 much longer, more pointed, and more recurved than is the case 

 with most land reptiles, indicating their use for the capture and 

 retention of slippery, quick-moving prey. 



The single-headed ribs are short, proving that the body was 

 slender, and doubtless cylindrical, more like that of a snake. The 



