PEOCEEDIlNrGS OF THE NATIONAL MUSEUM 



VOL. 73 



me. He insists that the peripheral bones are equivalent to the bones 

 of the marginal keels of Dermochelys and belong, therefore, to the 

 epithecal layer, while I have regarded them as belonging to the 

 deeper layer. In my original j)aper ^ I could rely only on the rela- 

 tion of the marginal scutes to the underlying bones of tortoises in 

 general and on certain bosses on the peripherals of Toxochelys to 

 sustain my view. Now, however, that these epithecal bones have 

 been discovered on the median and lateral keels of Chelys and on its 

 peripherals, I do not see how Doctor Volker or anybody else can 

 refuse to accept my identifications. It is evident that the theco- 

 phorous peripherals were not derived from the athecate marginals. 

 With the acce]3tance of this view Doctor Volker would be relieved 

 of his difficulty (his page 525) in explaining how it happens that the 

 horny scutes do not coincide with the peripherals. 



On his page 530, Volker concedes that the suprapygal bones belong 

 to the thecal skeleton, in view of my discovery that these in Toxo- 

 chelys were overlain by epithecal elements, but he insists that the 

 pygal bone is an epithecal bone. As he says, " Neither Demwchelys^ 

 nor Archelon, nor Protostega offers a solution of the question." To 

 this may be said that Chelys does offer the solution. This bone is 

 covered by the rear ends of the twelfth marginal scutes. As told 

 above, on the twelfth scute area of the right side is a large loose bone 

 (pi. 2, fig. 1) 20 mm. long. A few millimeters above it is another 

 small scale of bone. Near the upper left border of the pygal (same 

 figure) is tightly embedded an epithecal scale. 



In evaluating the affinity of Dermochelys with the Cheloniidae 

 Doctor Volker places the supposed epithecal marginal bones in the 

 balance in favor of a close relationship. If we accept his view the 

 elements of the shells of the Thecophora and of the Athecae may be 

 thus expressed (his page 630) : 



Thecal 

 ments 



ele- 



Epithecal ele- 

 ments 



Thecophora 



Nuchal. 

 Neurals. 

 Costal plates. 

 Plastral bones. 



Marginal bones. 

 Pygal. 



Vestigial shreds on the 

 keels of rare species. 



Thecal ele- 

 ments 



Athecae 



Nuchal. 



Possible shreds of cos- 

 tal plates on the 

 ribs. 

 .Plastrals. 



_ . , , f Dermal armor, upper 



Epithecal ele- ^^^ j^^^^.^ 



°^®"*^ [Marginal bones. 



If now the marginal elements and the pygal of the Thecophora 

 belong to the thecal layer, as shown above, the statement will stand 

 thus: 



•Amer. Naturalist, vol. 32, 1898, pp. 929-948. 



