242 



WATER REPTILES OF THE PAST AND PRESENT 



as distinguished from the Thecophora. Dr. Hay, whose authority 

 on fossil turtles is of the highest, believes that its line of ancestry has 

 been distinct from that of all other turtles from Triassic times at 

 least. Others believe that the leather-back is merely a highly 

 specialized form derived from the ordinary shelled type, a de- 

 scendant of some of the marine turtles of Cretaceous times. In 



Fig. 128. — Dermochelys coriacea. (From Brehm) 



support of the primitive ancestry of the leather-back Dr. Hay 

 offers the following: 



"The writer holds the view that the earhest turtles possessed 

 practically two kinds of shell, one purely dermal, consisting prob- 

 ably of a mosaic of small bones arranged in at least twelve longi- 

 tudinal zones. Each zone probably consisted of a row of larger 

 bones bordered on each side by smaller bones. Each of these bones 

 was covered by a horny scute. The nearest approach to such a 

 dermal shell is in our days seen in Dermochelys. Beneath the skin 

 there seems to have existed a carapace more or less complete, which 

 consisted of a nuchal, a median row of neurals, eight pairs of costals, 



