G. R. Wieland — Protostegan Plastron. 15 



Art. III. — The Protostegcm Plastron; by George R. 

 Wieland. (With Plate II.) 



Owing to the fact that the bones of the plastron of the 

 turtles from the older formations are usually fragile and are 

 made up of many parts, we have but few restorations of this 

 portion of the testudinate skeleton from pre-Tertiary species. 

 And so far as the gigantic turtles of the American Cretaceous 

 are concerned we lack a complete restoration entirely, though 

 Hay has given a partial restoration of Protostega gigas Cope.* 



The present contribution may in some degree fill up this 

 gap. It is based on two specimens of the turtle closely related 

 to Protostega, which I have described as Archelon ischyros* 

 both of which I collected near the Cheyenne River in South 

 Dakota from the very uppermost Fort Pierre Cretaceous. 



As these plastra, though fairly well preserved, were broken 

 into hundreds of pieces while yet in place, it has not been 

 thought advisable to indicate fractures. Nor has it been 

 deemed necessary to indicate in the illustrations that the exter- 

 nal spines of the left hyo- and hypo-plastron have been in part 

 determined from those of the corresponding elements of the 

 right side. The fact that all the figures given are from a 

 single specimen, as confirmed by a second, adds to their value. 



General Description. 



The Entepiplaston (see "measurements, p. 19). — The most 

 striking feature of the protostegan plastron is the distinct 

 fusion or lack of separation of the three anterior plastral ele- 

 ments, — that is, of the entroplastron and the two epiplastra. This 

 single element is a feature hitherto unobserved in the Testndi- 

 nata and may be described as the entepiplaston or paraplas- 

 tron. It is a rather heavy and a very shapely bilaterally sym- 

 metrical T-shaped bone, gracefully curving outward into two 

 broad als6 which slowly thin out to no more than the thickness 

 of cardboard along their posterior edges. Anteriorly the edge 

 is rounded except in the central portions, where there is a dis- 

 tinct downward and outward-looking external chamfer. At 

 each forward extremity of the alse there is one distinct spine 

 followed by a second which is flat and thin. The remainder 

 of the outer edge tends toward termination in thin spines. 

 While the outer surface is somewhat convex in general relief, 

 the inner is slightly concave and flat except for two lateral 

 longitudinal bosses about six centimeters long, one centimeter 

 high and twenty centimeters apart, anterior to which are 



* Field Columbian Museum Publication 7, Chicago, 

 f See this Journal for December, 1896. 



