238 WATER REPTILES OF THE PAST AND PRESENT 



row of large, median, supraneural elements render it quite certain 

 that there was a development of leathery hide comparable to 

 that of Dermochelys. The same may be said of Protostega gigas. 

 But Archelon Marshii had a less reduced carapace, and the leathery 

 skin was probably less well developed; and Protostega Copei, in 

 which no trace of supraneurals remains, must have made some 

 approach to the horn-shield condition. A more distinct suggestion 

 of transition from the leathery to the horny shield covering may be 

 seen in the very different contemporary Cretaceous form, Toxo- 

 chelys Bauri, where ossified epi- or supraneurals occupy quite 



Fig. 127. — SkuU of Archelon ischyros: pa, parietal; /, frontal; pm, premaxilla; 

 ^/, prefrontal; ^i/, postfrontal; m, maxilla, y,jugal; gj, quadratojugal; ^mj-, surangu- 

 lar; d, dentary; an, angular. (After Wieland.) 



exactly the nodal relation of the five vertebral horn shields of later 

 turtles, like Lytoloma, though there are not the slightest traces of 

 sulci. 



"From a purely anatomical standpoint I have suggested that 

 Archelon had seven dorsal keels corresponding to those of Dermo- 

 chelys. There is much excellent reason for regarding dermogene 

 ossification as essentially double-layered throughout the Reptiha. 



"In any restoration of Archelon ischyros only the mid-line should 

 be accentuated as a series of rather sharp supraneural crests. These 

 are shown to have been present by the characteristic groove-like 

 median pits with radiating striae, which are such a prominent 

 feature of epineurals. It is reasonable tb believe that the pits 

 mark the attachment of horny crests developed in the leathery 

 hide. Such were doubtless projected, more or less keel-like, to 



