318 DR. C. W. ANDREWS ON A NEW SPECIES OF 



to fill wp one of the gaps in the series of forms Which end in 

 Dermochelys. According to Dollo's * view, the Cheloniidse and 

 related forms, which no doubt were derived from littoral and, 

 more remotely, from terrestrial types, underwent a reduction of 

 their thecal armour in consequence of their becoming more and 

 more adapted to pelagic life ; this reduction seems to have 

 culminated in the later Cretaceous, in such forms as AUo2yleuron, 

 Protosteya, Archelon. Dollo then supposes that some such pelagic 

 form re-;vdopted a littoral or perhaps a swamp life and that these 

 new conditions rendered a protective armour again necessary. 

 This was not formed by a re-development of the thecal skeleton, 

 but by the formation of an epithecal armour external to it. 

 Whether, as Dollo seems to believe, this was an entirely new 

 formation or resulted from the increased development of epithecal 

 elements already present, as Volker, Hay, and others think, is not 

 certain. The presence of epithecal elements in the neural and 

 probably in the supramarginal regions of the shell of Archelon 

 gives some support to the latter view, especially as it is generally 

 agreed that the marginals of all Chelonians are of epithecal 

 origin. However this ma)' be, it appears that in forms like that 

 now described a very strong epithecal skeleton with a strongly 

 sculptured outer surface was developed outside the reduced thecal 

 shell, and this could only have been of use to a littoral or even 

 partly terrestrial animal. This stage in the series should theoreti- 

 cally occur in the eai-ly Tertiary period, and it is precisely from 

 this horizon (Lower or Middle Eocene) that CJosmGchelys comes. 

 It is supposed that subsequently this or some similar form 

 returned to a pelagic mode of life, which in turn resulted in the 

 reduction of the epithecal skeleton. In the Oligocene-Miocene 

 genus Psepliophoras, this has not advanced ver}- far, and is chiefly 

 manifested in the multiplication of the number of plates between 

 the ridges and in the less strongl}^ developed sculpture of the 

 surface. In the culminating form, Dermochelys, the reduction 

 has advanced so far that the plates of the carapace are very thin,, 

 smooth, and very numerous between the ridges : in the plastron 

 they have almost entirely disappeared. Unfortunately, in 

 Cosmochelys nothing is known of the plastron, either thecal or 

 ejiithecal, but probably the epithecal was well developed. 



The discovery of further remains of this interesting Chelonian 

 will be awaited witli great interest. 



* Dollo, " Preinievo Note suv les Clioloiiieiis oligocenes et iieogeiies de la 

 Belgiqne,' Bull. Mus. roy. d'Hist. Nat. de Belgique, torn. 5 (1888), p. 59. Also- 

 " Sur r Oiigiiic de la Toitue hnth {DermooJteli/s coriacea), " BuW. Soc. ro}'. des 

 Sciences inedicales et iiatuvelles, 1901. Also '" JSochehme hrahantica .... efc 

 rE.volution des Cheloniens maviiis," Bull. Acad. roy. des Belgique, 1903, p. 792, and 

 otlier i^apers. 



Lists of papers relating to the origin of the Athecae are given by Versluys, Report 

 Brit. Assoc. 1913 (Birmingham), p. 806; and by Volker, on " Ueber das Stamm, 

 Gliedmassen-, und Hautstelet von Dermochelys coriacea," Zoologische Jahrbiicher 

 ( Anatomic), vol. xxxiii. (1912-13), p. 543. 



