I. PLASTRON OF THE PROTOSTEGIN^. 



By G. R. Wieland. 



In all the earlier discovered skeletons of the huge Cretaceous tur- 

 tles included in the Protosteginae, the hyoplastra were found in 

 normal position in contact with the peculiar T-shaped entoplastron 

 characterizing this subfamily ; while elements definitely referable to 

 the epiplastra were singularly absent. This condition having repeated 

 itself at widely separated localities, and in two genera as represented 

 by fully six specimens approaching completeness, I was led to sup- 

 pose after the discovery of a median nuchal-like bone in Archelon 

 that the T-shaped entoplastron might represent a fusion of the epiplastra 

 with the entoplastron. This idea of fusion was beset by certain doubts 



at the time it was discussed in my descrip- 

 tion of the plastron of the type specimen 

 of Archelon, and has proven incorrect. 

 Dr. E. C. Case and Dr. O. P. Hay also 

 held unpublished odinions on the subject, 

 the one being inclined to accept, the 

 other disagreeing with the idea of a sup- 

 posable fusion of the anterior plastral 

 elements ; although both had mistakenly 

 identified the T-shaped entoplastron as 

 the nuchal of Protostega. However, as 

 presenting one more example of the exi- 

 gencies attending the uncovering of the 

 fossil record, in spite of the various speci- 

 mens known and the fact that the question 

 of plastral structure in the Protosteginae 

 had thus become an open one, no direct 

 evidence was obtained until two years ago. 

 Then I secured on the west bank of the Cheyenne River where it 

 breaks through the Oligocene Bad Lands of South Dakota, the greater 

 part of a large Archelon skeleton including along with the hyo-, hypo-, 

 and xiphiplastra a single epiplastron. This proves of quite unexpected 

 interest, because it is of the out-turned type seen in the Trionychids and 



FlG. I. Achelon ischyror 

 Wieland. Left epiplastron. 

 Inner (superior) view. X |- 

 c, anterior limit of entoplastral 

 overlap. Compare with epi- 

 plastra of Aspidonectes, etc., in 

 figures 2 and 3. 



