PROF. n. G. SEELEY ON PSEPH0PH0KU8 POLYGONUS. 409 



the vscven. longitudinal ridges which extend along it. The main median 

 ridge is most elevated posteriorly ; but the three lateral ridges on 

 each side maintain about equal elevation throughout their respective 

 lengths, and converge towards each other very slightly anteriorly, 

 and more markedly towards the posterior end. A specimen in the 

 British Museum has a length of a hundred and fifty centimetres ; 

 . the greatest transverse width round the curve of the back is more 

 than a hundred centimetres, though the width from side to side in 

 a straight line in the same position is not much more than seventy 

 centimetres. The bones which form the median ridges are often 

 three centimetres long, and abut against each other by transverse 

 sutures, much as in the fossil ; but the ridges in the recent ^pliargis 

 are sharper and more broken by irregularities, owing to the fact that 

 some plates have the ridge developed to a greater height than others. 

 In the wide part of the carapace, the transverse width between two 

 lateral ridges is about sixteen centimetres, and in this distance 

 fourteen or fifteen polj-gonal bones may usually be counted. Ante- 

 riorly the plates become rather smaller, and seventeen or eighteen 

 plates may be counted between two ridges ; and it is also here ob- 

 served that the transverse measurement of each plate is usually 

 greater than its longitudinal measurement. The plates mostly vary 

 from half a centimetre to over a centimetre in diameter. Some of 

 the plates appear to give indications of a slight radiated surface- 

 wrinkling, like that seen in some parts of the fossil ; so that, as 

 Midler first suggested and Yon Meyer afterwards asserted, the re- 

 semblance of Psephojphorus to Sphargis is close, and quite justified 

 Puchs in stating that it was only necessary to see the two types to 

 admit their general identity. In fact, there is nothing but difference 

 in the size of the plates to distinguish the two genera, as far as the 

 shield is concerned. It is, of course, impossible to say how many 

 longitudinal ridges may have extended along the carapace of Psepho- 

 jpJiorus. If there were many, the interspaces between the ridges 

 certainly wanted the concave character visible in the recent genus ; 

 and this apparent flatness of the carapace leads me to suspect that 

 in Fsephophorus the ridges may have been fewer than in Spliargis. 

 The slight longitudinal curvature visible in the median ridge in the 

 fossil, together with the slight transverse elongation of the bony 

 plates, would lead me to regard the specimen as beiug probably part 

 of a lateral ridge from the anterior portion of the shield, and not far 

 from its margin ; for what has appeared to be the under shield 

 formed of thin plates, imperfectly seen at its posterior end, where 

 many plates of the skeleton have been broken away, would well 

 correspond to that part of the carapace of Spliargis which is bent 

 round inferiorly at the outer sides of the shield ; so that when 

 fossilized, these portions might then present the appearance of two 

 shields superimposed on each other. 



It will readily occur to any one familiar with the Chelonia that 

 the type which is represented by Spliargis and Psepliopjilioriis, differs 

 from all the other members of the order, not only in having the ribs 

 entirely separate from the carapace, but also in the fundamental 



