o SPHARGIDJE. 



The ccrvicals are short and similar to those of the Chclouida). All 

 the dorsal ribs except the last articulate with the ueural arch aud 

 two centra ; the first and tenth ribs are short. The caudal vertebrae 

 are i)roeoclous. Chevron bones are absent. 



The dorsal shield consists of a mosaic of extremely numerous 

 polygonal bony plates of unequal size, the largest forming seven 

 longitudinal keels or ridges ; it is produced into a pointed supra- 

 caudal portion, which covers the tail. In the young the dorsal 

 shield is soft and leathery. Between the shield and the last cervical 

 vertebra a bone is present which is the homologue of the nuchal of 

 other Chelonians, 



The ventral shield is similar to and continuous with the dorsal, 

 and likewise bears ridges of enlarged tubercles ; in the young both 

 shields are extremely similar, but, with age, they differ in the ventral 

 remaining soft, only the tubercles of the longitudinal ridges and 

 a few others being bony. There is an underlying annulus of slender 

 bones, eight in number — the plastral elements — viz. the epiplastra, 

 the hyoplastra, the hypoplastra, and the xiphiplastra ; an ento- 

 plastron or interclavicle is absent. 



The skull bears a certain resemblance to that of CJielone, which 

 is, however, limited to the shape and the general constitiition of the 

 temporal roof; in the absence of the column-like processes of the 

 parietals descending to the pterygoids in front of the supraoccipital 

 and the prootics, and in the 02)i.sthotic not coming in contact w ith 

 the squamosal, it differs from that of all other Chelonians. 



The external bony nostril is very large and situated entirel}- upon 

 the upper surface of the skull, in front of the orbits ; the choante 

 are quite anterior, the nasal duct being nearly vertical, and are 

 separated by the vomer, which also separates the ])alatincs. The 

 pra'niaxlllaries are distinct; the praefrontals, which join the post- 

 frontals, form a very short suture in front of the frontals. In one 

 of the adult skulls in the British Museum, the lachrymals are nearly 

 comj)letely separated from the maxillaries ; but tliis is merely an 

 individual peculiarity. The parietals entirely or nearly entirely 

 cover the supraoccipital ; they form a long suture with the post- 

 froutals, and a short one with the squamosals. The lower border 

 of the postfrontal joins the jugal and the squamosal, and, contrary 

 to what exists in the Chelonido), is separated from the quadratt)jugal 

 by the two latter bones. The quadrate bone is a good deal within 

 the outer border of the skull, and separates the opisthotic from 

 the squamosal; the ear-chambers are open behind. The foramen 

 magnum, which is deeper than broad, is bordered by the basi- as 

 well as by the supra- aud exoccii)itals. The pterygoids are smaller 

 than in any other Chelonian ; they are nearly entirely separated 

 from each other by the large basisphenoid, which extends forwards 

 as far as the posterior borders of the orbits, and widely separated 

 from the maxillaries by the palatines ; their lateral borders are 

 nearly straight, and without " ectopterygoid " processes ; in front, 

 they are in contact with tlie largo ])alatines, behind with the 

 quadrate and the basioccipital. The mandible has the same struc- 



