G. JR. Wieland — Protostegan Plastron. 17 



anterior plastral elements as in Tholassochelys, etc., the plastron 

 would probably have had a greater length than the carapace. 

 7. There is an element which I figure and think must be 

 the nuchal. It resembles markedly the nuchal of Sphargis 

 and corresponds closely to the bone which Cope believed to be 

 the nuchal of Protostega gigas, (Cretaceous Yertebrata of the 

 West, plate XII, fig. 11.) The figure and description is 

 given herewith. The nuchal (see meas.) is a flat fan-shaped 

 bone distinctly incurved anteriorly, and spreading out into 

 two broad alee projecting well forward. These must have con- 



Nuchal. Under or internal side. One-tenth natural size. 



nected with the marginalia, the anterior pair of which were 

 quite thin, fragments having been recovered. It should be 

 remarked that the thickness laterally and also of the thin pos- 

 terior fan, the edge of which is slightly notched as if for 

 sutural union with the first neural, as well as striation, all favor 

 the nuchal position for this bone. The latter is especially true 

 of a small dorsal pitting from which radiate several distinct sur- 

 face striations corresponding to those radiating from the median 

 neural groove* of the carapace. The inferior side is marked by 

 a prominent centrally-placed elevation two centimeters in height, 

 having the shape of an isosceles trapezoid distinctly buttressed 

 at the four corners. The long and anterior base of this trape- 

 zoidal ridge has twice the length of the shorter, and a slight 

 central elevation is enclosed. This elevation must have formed 

 a surface for articulation with the last cervical vertebra. This 

 element evidently formed no part of the plastron, and as I have 

 previously described the pygal I can suggest no other than the 

 nuchal position. (See Figure 2.) 



Hay, in his publication previously mentioned, figures a por- 

 tion of the entepiplastron which he says must be regarded as 

 the nuchal. He mentions the grooves of the [upper] surface 

 as internal, that is, facing downward, and presumably repre- 

 senting contact with transverse processes of the last cervical 



* See description of carapace, this Journal, Dec. 1896, page 400. 

 Am. Jour. Scr. — Fourth Series, Yol. Y, No. 25.— January, 1898. 



