CHELONE GIGAS. 3 



same surface in existing Turtles. From the end of the occipital condyle to the poste- 

 rior border of the palatonaris the extent is nine inches in Chelone gigas. The palatal 

 opening is, as in Sphargis, more distinctly divided or indicative of the two nasal passages 

 than in Chelone my das. 



The least breadth of the suturally united pterygoids dividing the lower temporal 

 openings is two inches ten lines ; in Chel. mydas it is one inch five lines. 



The less perfect skull of Chelone gigas obtained in 1858 is represented chiefly by the 

 upper wall, to the outer surface of which is cemented portions of the scapulo-coracoid arch. 

 On clearing out the matrix from the inner or under surface of this specimen parts of the 

 side walls of the cerebral cavity and of the orbits, with the alveolar border of the upper 

 jaw, two inches and a half in advance of the orbit, were brought into view. The 

 comparable parts confirm the affinity to the true Turtles, Chelone, as contrasted with the 

 " leatherbacks " (Sphargis). 



The extreme breadth of this cranium is . . 13 inches. 



The preserved length . . . . 15 „ 



The extreme breadth of the cerebral chamber . 2 „ 



The portion of the alveolar border testifies, as in the better preserved specimen, to 

 the Chelonial character. 



The decisive test of the affinities of the Eocene gigantic representative of the marine 

 family of the Order Chelonia is afforded by the portion of the petrified carapace and 

 plastron associated with the later discovered skull. The Leatherback Turtles (Sphargis) 

 have little of the carapace besides the normal unexpanded pleurapophyses (dorsal ribs) 

 to undergo the conservative petrifying process. 



The expanded horizontal plates from the summits of the neural spines and the 

 corresponding expansions from the upper surface of the pleurapophyses, constituting the 

 so-called neural and costal plates, have been preserved in eight portions of those modified 

 segments of the carapace of Chelone gigas which have been recovered. 



The neural plate answering to the ' sixth ' of the entire carapace measures four 

 inches in length and eight inches in breadth ; the lateral halves slope with a feeble 

 concavity from a slightly elevated medial rising as in the hinder plates of Chelone 

 subcristaia} The costal sutural border makes a very low angle at about the same 

 distance from the anterior border as in the corresponding plate of Chelone longiceps. 2 

 The marginal thickness of the plate is nine lines. The neural plates are relatively 

 broader in proportion to their length than in any recent or hitherto observed fossil 

 species of Chelone. 



About one inch of the first neural plate and three inches of the seventh are preserved 



1 ' Monograph on the Fossil Eeptilia of the London Clay,' Part I, Chelonia, issued by the Palseonto- 

 grapbical Society, for the year 1848, p. 24, PI. VIII, fig. 1, 1849. 



2 lb., ib., p. 16, PI. IV, fig. 2, s 6. 



