ON PROTOSTEGA, THE SYSTEMATIC POSITION 
OF DERMOCHELYS, AND THE MORPHOGENY 
OF THE CHELONIAN CARAPACE 
AND PLASTRON. 
i, ee NAY. 
Tue structure and relationships of the genus of fossil tur- 
tles known as Protostega are being gradually determined. 
Important additions to our knowledge regarding it have been 
made recently by Dr. E. C. Case! and Mr. G. R. Wieland.? 
The former describes and figures in an excellent way the 
plastron, the skull, the shoulder girdle, and the limbs ; and 
discusses at length the relationships to the other Testudines. 
Mr. Wieland supplies needed information regarding the ribs 
and the existence of neuralia. He regards the form which he 
describes as a new genus, which he calls Archelon, but it will 
be generally agreed, I think, that it is not distinct from Pro- 
tostega. 
One of the most important discoveries made by Case is the 
arrangement of the xiphiplastrals. When I wrote my paper® 
on the portion of the plastron of this animal then in my hands, 
I assumed that the xiphiplastrals had essentially the same form 
and dimensions as in the modern genus Thalassochelys. Case 
finds that, on the contrary, immediately after these plastral 
elements have freed themselves from the hypoplastrals they 
sharply curve toward the mid-line and come into contact. The 
length of the plastron is thus much reduced. Dr. Case also 
concludes that the epiplastrals must have been much shorter 
than they are in Thalassochelys, and that the entoplastron was 
probably wanting. From this condition of the plastron Case 
concludes that my estimate of the size of Protostega was much 
1 Journ. of Morphology, vol. xiv, pp. 21-55, Pls. IV-VI. 
2 Amer. Journ. Science [4], vol. ii, pp. 399-412, Pl. VI, and 19 text-figures. 
3 Field Columbian Museum Pubs. Zoology, vol. i, pp. 57—62, Pls. IV, V. 
