14 BRITISH FOSSIL REPTILES. 



The plastron of the Chelone brcviceps (PI. 16, fig. 2), although more ossified than in 

 existing Chelones, yet presents all the essential characters of that genus. There is a 

 central vacuity left between the hyosternals {lis)* and hyposternals {ps); but these 

 bones differ from those of , the young Emi/s in the long pointed processes which radiate 

 from the two anterior angles of the hyosternals {hs), and the two posterior angles of the 

 hyposternals {ps)* 



The xiphisternals [xs) have the slender elongated form, and oblique union by 

 reciprocal gomphosis with the hyposternals {hs), which is characteristic of the genus 

 Chelone. The posterior extremity of the right episternal {es) presents the equally 

 characteristic, slender pointed form. 



With these proofs of the modification of the plastron of the present fossil according 

 to the peculiar type of the marine Chelones, there is evidence, however, that it differs 

 from the known existing species in the more extensive ossification of the component 

 pieces ; thus the pointed rays of bone extend from a greater proportion of the margins 

 of the hyosternals and hyposternals ; and the intervening margins do not present the 

 straight line at right angles to the radiated processes. 



In the Chelone mi/das, and Chel. caouanna (fig. 3, p. 4), for example, one half of 

 the external margin of the hyosternal and hyposternal, where they are contiguous, 

 are straight, and intervene between the radiated processes, which are developed from 

 the remaining halves, while in the Chelone hreviceps, about a sixth part only of the 

 corresponding external margins are similarly free, and there form the bottom, not of an 

 angular, but a semicircular interspace. 



The radiated processes from the inner margins of the hyosternals and hyposternals, 

 are characterised in the Chelone breviceps by similar modifications, but their origin is 

 rather less extensive ; they terminate in eight or nine ra3^s, shorter, and with intervening 

 angles more equal than in existing Chelones. The xiphisternal piece, xs, receives in a 

 notch the outermost ray or spine of the inner radiated process of the hyposternal, as 

 in the Chelones, and is not joined by a transverse suture, as in the Emydes, whether 

 young or old. 



Subjoined are dimensions of the plastron of Mr. Bowerbank's fossil : 



Inches. Lines. 

 Shortest longitudinal diameter of hyosternal and hyposternal pieces 2 .5 



Transverse diameter of ditto ...... 1 7 



Total length of plastron ....... 6 



The bones of the scapular arch, especially the coracoid, Cuvier has shown to afford 

 distinctive cliaracters of the natural families of the Chelonia ; but the Eocene Chelonites 

 described Jjy Cuvier, did not yield him this opportunity of thus testing their affinities. 

 In the Chelone breviceps here described, the left coracoid (shown in fig. 2) is jireserved in 

 nearly its natural position ; it is long, slender, symmetrical ; cylindrical near its humeral 



* These letters refer to the parts in the typical plastron, fig. 3, p. 4, by reference to which tlic homo- 

 logous parts in the fossil will be readily understood. 



