184 RECORDS OF THB AUSTRALIAN MUSEUM. 



convex anteriorly. In C. depressa (fig. 47b) this ridg-e is A- 

 shaped and is more distinctly marked on the basi-pterj-goid 

 process of the bone (processus hasipterygoideus ossis basispheuoid 

 of Dr. Kesteven*^). In this respect it bears a closer resem- 

 blance to all the other recent turtles than to Chelouia hu/diis. 

 Ventrally the basioccipital has a strong median sulcus (fig. 

 47b) as in Colpocltehjs /re/»p//, Garman, and the fossil CJtelone 

 cuiiei'ceps of Owen. Regarding the latter Owen writes'*^ — "The 

 basioccipital is remarkable for the strong development of the 

 tubercles for the insertion of the 'recti capitis antici' and for 

 the depth of the groove between them." This describes almost 

 exactly the condition exhibited by C. depressa, which resembles 

 that of the bastard-turtle more than any other recent form. 



In ray specimen of C. depressa (fig. 47 and 48a) there is a 

 slight but distinct reduction in the extent of the temporal 

 roof. This is not due to a reduction in the length of the 

 parietals but to an emargiuation in the region of the parieto- 

 squamosal suture, which allows the somewhat larger post- 

 frontal of C. depressa to actually extend slightly behind 

 the deepest part of this bay. The oldest known fossil 

 Chelonians possessed a well developed temporal roof, the 

 significance of which has been masterfully explained by Dr. 

 Hay ,-^3 -^vho, with Baur, Cope, and others, i-egard it as probably 

 handed down to them by their Cotylosaurian ancestoi'S. These 

 archaic Cheloneformes also possessed true nasal and lacrymal 

 bones, which no longer persist in living marine turtles, though 

 they are present as separate ossifications in some living mem- 

 bei's of the order. We find the recent Chelonida^ then, to be a 

 peculiar mixture of the retention in some cases, and loss in 

 others, of their most primitive characteristics.'^ If this 

 character holds good in the adult skull it will be very interesting 

 to note this slight reduction of a typically primordial condition 

 in a living form, a specialisation suggested by Dr. Hay^ to l)e 

 correlated with the evolution of the Pleui'odiran neck. 



•»' Kesteven — Proc. Rov. Soc. N.S. Wales, xUv., 1910, pi. xxviii., 

 fig. 42. 



■»■; Owen— Hist. Brit. Foss. Reptiles, i., 1849-1884. p. :?:^ 



« Hay — Foss. Turtles N. Anier., Carnegie Inst., publ. 75, 1908, p. 19. 



« Dr.' Gadow (Cambr. Nat. Hist. Reptiles. 1901, p. ;?80) holds the 

 opposite view, tliat the Chelonidaj are a specialised offshoot of the 

 Cryptodira and that there is nothing primitive about them, except the 

 complete series of inframarginal sliields. 



*^ Hay— Foss. Turtles N. .Amer., Carnegie Inst., publ. 75., 1908. p. 19. 



