CHELONIA. 19 



plate (^i) ; in which it resembles the Cliel. my das. Both posterior angles of the first 

 neural plates are produced, and truncate to articulate with the second pair of costal 

 plates ; and the second neural plate is quadrangular. In a portion of another carapace 

 of the Chelone lo7igiceps the second neural plate (52) is pentangular, the right 

 anterior corner being produced, and truncate to join with the first costal plate of the 

 right side ; the left posterior corner of the first neural plate (-91) being produced, and 

 truncate, to articulate with the second costal plate of the left side. This structure 

 I believe, however, to be an individual variety. But the characters of the species are 

 exemplified in more constant modifications of the carapace. The succeeding neural 

 plates to the seventh inclusive (53 — 8^^ are hexagonal, with the anterior lateral 

 border much shorter than the posterior lateral border, as in Chelone mydas, and not of 

 equal extent, as in Chelone breviceps ; they become more equal in the seventh and 

 eighth neural plates, which also decrease in size ; the ninth plate (59) is very small, 

 quadrangular, and oblong, as in Mr. Lowe's fragment. Only a small portion of the 

 tenth neural plate is preserved in Mr. Bowerbank's beautiful specimen. 



The impressions of the horny scutes are deeper, and the lines which bound the 

 sides of the vertebral scutes {v\ — va) meet at a much more open angle than in the 

 Chel. breviceps, in which the vertebral scutes have the more regular Jiexagonal form of 

 those of the Chel. mydas. Their relations to the neural plates are nearly the same as 

 in Cliel. breviceps. 



The plastron (Tab. V, fig. 2) is more remarkable than that of the Chel. bremce2)s 

 for the extent of its ossification ; the central cartilaginous space being reduced to an 

 elliptical or subquadrangular fissure. The four large middle pieces hyosternals {hs) 

 and hyjjosternals (ps), have their transverse extent relatively much greater as compared 

 with their antero-posterior extent, than in the Chel. breviceps; and this might be 

 expected, in conformity with the broad character of the bony cuirass indicated by the 

 carapace. The median margins of the hyosternals {hs) are developed in short toothed 

 processes, along their anterior three fourths ; the median margins of the hyposternals 

 (ps) have the same structure along nearly their whole extent ; the intermediate space 

 between the smooth or edentate margins of the opposite bone is ten lines ; the 

 expanded end of the long coracoid (52) is seen projecting into this space. 



The xiphisternals {xs) are relatively broader than in Chel. breviceps, or in any of 

 the existing turtles ; and are united together, or touch each other, by the toothed 

 processes developed from the whole of their median margins. The entosternal piece is 

 broad, flat on its under surface, and is likewise dentated at its sides. 



The outer surface of each half of the plastron inclines, as in the Chelone mydas, 

 towards a submedian longitudinal ridge. 



The breadth of the plastron, in the specimen figured (fig. 2), along the median 

 suture, uniting the hyosternals and hyposternals, is six inches : the narrowest antero- 

 posterior diameter of the conjoined hyosternals and hyposternals is two inches nine lines. 



