ORDER CHELONIA. JO89 



genus Psephoderma be rightly referred to it, we have at once a great 

 obstacle to the acceptance of his view. 



The suborder may be briefly characterised by the circumstance 

 that the carapace is entirely of dermal origin, and quite separate 

 from the vertebrae and ribs, and may consist merely of a series of 

 marginal bones, or of marginals with a single median dorsal row of 

 broad scutes, or of a number of small irregular scutes, with longi- 

 tudinal rows of larger ones; while the plastron (fig. ion) has no 

 entoplastral (interclavicular) element. The cranium is characterised 

 by the absence of vertical plates connecting the parietals with the 

 pterygoids. All the forms are of marine habits, and consequently 

 the extremities of the limbs are modified into paddles like those of 

 the Chelonida. 



Before noticing the two established families, it may be observed 

 that the imperfectly known Psephoderma of the Upper Trias of 

 Bavaria and England is founded on a specimen which appears to 

 be a carapace of a member of this suborder, although it has been 

 suggested that it is not Chelonian at all. This presumed carapace is 

 formed of a number of polygonal scutes, traversed by longitudinal 

 rows of keeled scutes. Here also may be mentioned the genus 

 Macellognathus, founded upon the anterior portion of a toothed 

 mandible from the Upper Jurassic of North America, which Pro- 

 fessor Marsh regards as showing affinity with the Chelonia, and 

 which may possibly indicate a generalised family of the present 

 suborder. 



Family Protostegid^:. — This family is usually regarded as the 

 least specialised of the two that are yet established, although an 

 opposite view is taken by Dr Baur. The carapace, according to 

 the interpretation of that authority, is represented merely by a row 

 of marginal scutes ; but the plastron is strongly developed, and 

 composed of very thick ossifications. The type genus Protostega 

 occurs typically in the Cretaceous of North America. It was con- 

 sidered by its describer Professor Cope to have possessed a solid 

 carapace, but the bones which he regarded as probably dorsal appear 

 to belong to the plastron. It was also suggested that the dorsal 

 vertebrae were proccelous, with traces of transverse processes ; but 

 these vertebrae are probably referable to the cervical region. The 

 type species attained very large dimensions. An allied form from 

 the Upper Cretaceous of Italy has been described under the name 

 of Protosphargis (fig. ion), but further evidence is required to 

 prove its right to generic distinction from the American form. It 

 has, indeed, been asserted that there were no marginal bones, 

 but according to Dr Baur this is incorrect. In the Cambridge 

 Greensand and the English Chalk there occur humeri of Athecate 

 Chelonians which have been provisionally referred to Protostega. 



