16 



G. R. Wieland — Protostegan Plastron, 



small furrows. The long medial posterior or entoplastral 

 process is, however, distinctly triangular for the first third of 

 its length, and for the remainder quite round. The entire 

 surface of this bone is striated like the remainder of the 

 plastron, and not like the carapace. 



Entepiplastron. Upper or internal side. One-tenth natural size. 



That we have in this bone a true plastral element was de- 

 cided from the following reasons and facts : 



1. Its size and particularly its form will permit the supposi- 

 tion that it represents the entoplastron and epiplastra. 



2. There is the possibility that the epiplastra or their homo- 

 logue could lie athwart the anterior extension of the hyoplas- 

 trse, as is distinctly the tendency in Sphargis. 



3. I have twice seen this bone in place, and each time it 

 rested directly upon the anterior portions of the hyoplastra 

 and beneath numerous other skeletal parts, while the angular 

 edge of the entoplastral process faced upwards, and therefore 

 inwards as would be expected. Rounded edges and flat faces 

 would probably be exterior. 



4. The chances were in favor of the preservation of other 

 plastral elements, rather than of such a free nuchal as this bone 

 would have constituted, had they existed. Especially was this 

 the case in specimen ISTo. II, where the anterior parts were 

 quite undisturbed. 



5. There is no process for articulation with the last cervical 

 vertebra as in all living marine testudinates. 



6. Were this bone to be regarded as the nuchal, and the 

 remainder of the plastron then to be added to proportionally for 



