52 CHELOKTA. 



nares in hinder half of cranium, and in the adult of the larger 

 species situated very far back ; vomer as in Argilloclielys. Mandible 

 (figs. 8, 9) with very deep masseteric fossa, and long, depressed, and 

 wide symphysis, which in the adult of the larger forms is much 

 flattened both above and below. Carapace rounded posteriorly; 

 vacuities of shell still more obliterated than in Thalassochelys ; 

 epiplastrals narrow ; exposed portion of entoplastral very short ; 

 xiphiplastrals uniting extensively in the middle line, and often of 

 great relative width. 



Coracoid of the adult short and much expanded distally; but 

 longer and more slender in the young. Humerus (fig. 6) of the 

 type of Thalassochelys, but the head often placed still more obliquely 

 to the long axis, the shaft more constricted, the ulnar process rising 

 only a short distance above the head, and the radial process con- 

 nected more or less with the head. Terminal phalangeals not 

 flattened, and clawed. Neural bones of carapace comparatively 

 short. 



The synonymy has been discussed by the present writer 1 , who 

 has shown that the name Euclastes, which Dollo 2 proposes to adopt 

 in lieu of Erquelinnesia, is preoccupied. Lytoloma, which Dollo first 

 identified with the latter, is adopted as being the second earliest of 

 the terms based on the evidence of the skull. Osteopygis, which 

 is of earlier date, was applied to a shell, which may be generically 

 the same. The genus Puppigerus was founded on the "Eocene 

 marine turtles of the London Clay, and a Miocene species of North 

 America"; and since the so-called Chelone longiceps is the first 

 mentioned of the English forms there can be no hesitation in taking 

 that species as the type. The young of L. crassicostatum shows that 

 Puppigerus, as thus typified, must be regarded as a synonym of 

 the present genus. 



The skull in the larger forms bears a still greater size relatively 

 to that of the shell and limb-bones than in Thalassochelys. In the 

 adult, at least of the larger forms, the posterior nares become more 

 and more backward as age advances ; this development being 

 accompanied by an increase in the length of the mandibular sym- 

 physis, which at the same time becomes more and more flattened. 

 The upward direction of the orbits and the narrowness of the in- 

 terorbital bar also increases with age. 



The variation in the number of costal bones, on the evidence of 

 which the family Propleuridce (in which the present and allied types 



1 Quart. Joum. Gee Soc. vol. xlv. p. 233 (1888). See also Proc. Zool. Soc. 

 1889, p. 62. 



2 G-eol. Mag. dec. iii. vol. ?. ^p. 201-267 (1888) ; and Ann. Soc. Geol. Nord, 

 vol. xv. pp. 114-122 (1888). 



