FAUNA OF THE GOSAU FORMATION. 695 



to the Cholonia; for these fossils, if rightly interpreted, indicate an 

 animal with a relatively larger vertebral column than any Chelonians 

 now known, or at least with a vertebral column constructed ivpon a 

 different plan. The ribs were wide, as in the marine Chelonia, and 

 extended distally far beyond the limits of the plates which covered 

 them. The superimposed plate is developed chiefly behind the rib; 

 its anterior margin is smooth and rounded ; the posterior margin is 

 not preserved, but obviously became thin. The free articular end 

 of the rib was massive ; and the superimposed plate extended beyond 

 it proximaliy for an unusual distance, indicating great width for the 

 intervening vertebra. This portion of the dermal plate, which extends 

 mesially beyond the rib, has the external surface well preserved, but 

 wants the smooth ossified internal surface ; and as this is absent in 

 both specimens, it is possible that in the living animal this internal 

 part of the plate may have been cartilaginous or united to other 

 bones. Neither of the specimens gives the slightest indication either 

 of external scutes or ornament, or of union to adjacent bones; and 

 the external surface is such as would suggest that the bones were 

 probably contained, if not beneath a muscular covering, at least 

 beneath a skin which had not become specialized ; so that we have 

 here an animal that in some respects recalls the Protostega of Prof. 

 Cope, but differs essentially in the dermal plates being blended 

 with the ribs. An allied but undescribed type from the Cambridge 

 Greensand also has the costal plates separate from each other, but 

 differs in having them marked with scutes. 



A short description of each of these specimens appears neces- 

 sary. First, the less perfect of the two (PI. XXX. fig. 15) 

 shows a smooth external surface gently convex in length and 

 somewhat convex from side to side. It consists of a thick dense 

 dermal plate superimposed upon a rib, this plate probably being 

 a representative of the supracostal cartilages and ossifications 

 found in birds, crocodiles, and Batteria. Immediately above the 

 articular expansion of the rib, at the proximal end, the plate is a 

 centimetre thick, and is defined by the density of its texture from 

 the osseous matter of the rib beneath. This bone on the interior 

 surface is much eroded, but presumably extended much further to- 

 wards the median line of the animal's body, since the dermal plate 

 is prolonged with a rough under surface, due to this bone having 

 adhered to the bone with which it is blended; and as the plate is 

 prolonged mesially, its thickness becomes reduced to one half, though 

 the fractured specimen is imperfect at its margin. The transverse 

 width of the plate, as preserved above the expanded head of the rib, 

 is about 9| centim. About 9 centim. further away from the 

 middle line of the animal, a length of about 4 or 5 centim. of the 

 margin of the plate is rounded ; and here its union with the thin 

 underlapping, transversely expanded margin of the rib is distinctly 

 seen. On the opposite or posterior margin there are some faint 

 indications of a lateral rounding of the margin of the dermal plate. 

 At 18 centim. from the proximal end of the rib the dermal plate 

 comes to an end, being broken away, and .allows the rib to project 

 freely from under it, showing, I think, that although there was 



