IX ATHECAE SPHARGIDAE 335 



species were dying out. Perhaps they are very shy, leading a 

 pelagic life, diving at the least sign of danger, and coining near 

 the land only for the sake of breeding. 



The structure of Spliargis is so peculiar in many respects that 

 it deserves a somewhat full account. The neuro-central sutures 

 persist on aR the vertebrae. The eight cervicals are short. All 

 the ten trunk -vertebrae carry ribs, and these, with the exception 

 of the last, articulate between the centra and with the neural 

 arches ; the first and tenth ribs are short, the others are long 

 and flattened, but not broad, with wide spaces between them. 

 The tail is short, although it consists of about twenty vertebrae ; 

 these are devoid of chevrons. 



The skull superficially resembles that of Chelone, chiefly owing 

 to the" completely roofed-in temporal region. The supraoccipital 

 crest is rather short, covered completely by the parietals, the 

 posterior margin of which is rounded off instead of forming, as in 

 the Chelonidae, a long projecting triangular crest with the supra- 

 occipital. The parietals are in broad contact with the post- 

 frontals, posteriorly they are just reached by the squamosals. 

 The quadrato-jugal is small, separated from the post-frontal by the 

 meeting of the squamosal with the jugal. The quadrate is 

 notched behind, and it separates the opisthotic from the 

 squamosal. The basisphenoid is large and broad, extending far 

 forwards so as to separate the pterygoids widely from each other 

 except in their anterior portions, which, instead of sending a 

 lateral arm to the jugal and maxillary, as in Chelone, are widely 

 separated from these bones by the palatines. The choanae lie on 

 either side of the anterior half of the vomer, and are not roofed 

 over by ventral vomero-palatine wings. 



The limbs and their girdles are essentially like those of the 

 Chelonidae, but are not derivable from them. The most re- 

 markable feature is the shell. The dorsal and ventral halves are 

 directly continuous^ forming one unbroken case all round, which 

 is composed of many hundreds of little bony plates, irregularly 

 polygonal, fitting closely into each other with their sutural edges, 

 and giving the shell a beautiful mosaic appearance. On the 

 dorsal side are a median row and three pairs of lateral rows of 

 larger plates, and these form seven longitudinal blunt ridges 

 which all converge towards the triangularly pointed tail-end of 

 the shell. The ridges are not so much produced by thickened 



