CHELONIA. 19 



plate (2) ; in which it resembles the Cliel. wi/Jas. The second neural plate (3) is 

 pentangular, the left anterior corner being produced, and truncate to join with the 

 first costal plate of the left side ; the right posterior corner of the first neural plate (2) 

 being produced, and truncate, to articulate with the second costal plate of the right 

 side. This structure I believe, however, to be an individual variety. In another 

 carapace of the Chelone lon(jiceps, e. g. both posterior angles of the first neural plate 

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

 the second neural plate is quadrangular. But the characters of the species are 

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

 plates to the seventh inclusive (4 — 7) are hexagonal, with the anterior lateral border 

 much shorter than the posterior lateral border, as in Chelone mi/das, and not of 

 equal extent, as in Chelone hreviceps ; they become more equal in the seventh (y) and 

 eighth (9) neural plates, which also decrease in size ; the ninth plate (10) is very small, 

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

 last 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 {vy — v-\) meet at a much more open angle than in the 

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

 those of the Chel. mi/das. Their relations to the neural plates are nearly the same as 

 in Chel. breviceps. 



The plastron (PI. 13, fig. 2) is more remarkable than that of the Chel. brevicem 

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

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

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

 with their antevo-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 (lis) are developed in short toothed 

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

 (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 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 jilastron 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 



