PROTOSTEGIDJE. 201 



Each of the peripherals, except the posterior three, may be said to be V-shaped in trans- 

 verse section, one limb of the V extending toward the edge of the plastron, the other inward 

 and upward toward the costal plate. The upper plates are slightly concave above; the lower 

 ones somewhat convex below. The concave and the convex surfaces of each peripheral come 

 together to form a sharp edge, and the border of the carapace so formed runs from the fourth 

 peripheral to near the pygal. The three posterior peripherals have no lower, or plastral, 

 plate developt. What is regarded as the tenth peripheral has a half-pit at its hinder end. 

 It is therefore believed that the rib-end of the eighth costal was swung backward to be inserted 

 between the contiguous ends of the eleventh and the twelfth peripherals. In Caretta this rib- 

 end has moved backward still further and is inserted in the middle of the last peripheral. 

 The antepenultimate peripheral has a pit for the rib-end of the seventh costal near its 

 hinder end and this pit lies partly in the anterior end of the penultimate peripheral. If the 

 eighth costal plate is regarded as sending its rib to the last peripheral, the next peripheral in 

 front has no rib-end corresponding to it. In Caretta it is the antepenultimate peripheral 

 which receives no rib. 



The peripherals are smallest posteriorly, and they increase in size as far forward as they 

 are represented. The fourth, the most anterior one present, has its two plates standing at an 

 obtuse angle with each other; the fifth has them at nearly right angles, the others at less than 

 a right angle. The plastral plate of the fifth is 37 mm. wide, the carapacial plate is 30 mm. 

 wide. Two long narrow peripherals accompany the specimen, but they appear to be intrusive 

 and to belong to Toxochelys. Two neurals (fig. 257) are present, probably the third and the 

 fourth. The more anterior is 39 mm. long and 28 mm. wide; the other 30 mm. long and 25 

 mm. wide. The middle line of each rises into a low carina, which on the more anterior one 

 rises into a tubercle. The hinder end of the anterior neural is crost by a narrow dermal 

 sulcus, a fact which shows that dermal scutes were present. The peripherals also are crost 

 by sulci, but these are broader than that on the neural. 



The pygal is a rather peculiar bone, being nearly square, thick, convex on the hinder, 

 or upper, surface, and with a thick free border. It is 27 mm. high, 33 mm. wide and 11 mm. 

 thick. 



With the remains described is found a detacht median dorsal tubercle exactly like those 

 occurring in the dorsal carina of Toxochelys; but it is regarded as foreign to the specimen, 

 having probably been introduced with the peripherals just mentioned. 



Fig. 258 represents a costal plate of this species. It will be seen that it is less completely 

 ossified than the costals of our modern sea-turtles, but more completely than in even a large 

 Protostega gigas. In the figure the distal end is restored in outline from another costal present. 

 Measured thru the middle of the width the costal is 7 mm. thick, but near the sutural 

 border it becomes reduced to 3 mm. The costal figured is probably the fifth of the right side. 

 On it are found the sulci which separated two vertebral scutes from each other and from a 

 costal scute. These sulci are narrow and shallow. It is evident that the vertebral scutes 

 were relatively very broad, about 1 10 mm., nearly equal to one-third the width of the carapace. 

 There are present two bones which are referred to the skull; they are the two postfrontals 

 (fig. 259). Each is 30 mm. wide and 60 mm. long. At the hinder end the bone thins down 

 to a sharp, nearly smooth edge; so that it becomes probable that this edge formed the hinder 

 border of the roof of the temporal region; and hence, that the squamosal did not join the 

 parietal. The roof was probably not so extensive as in Caretta. 



In the Cope collection of the American Museum of Natural History is the left humerus 

 of a small sea-turtle which was collected in Gove County, Kansas, in 1877, by one of Professor 

 Cope's collectors. This humerus has every appearance of being that of an adult turtle. It has 

 suffered no compression or distortion whatever. Its length, from the proximal surface of the 

 head to the distal end of the bone, is 64 mm. The ulnar process rises slightly above the head. 

 The radial process descends 35 mm. below the proximal surface of the head. This process 

 is 11 mm. thick. The width of the humerus across this process is 23 mm. Just below the 

 process the width is 16 mm. ( )n the upper surface of the bone, opposite the radial process, 

 is a deep nluscular scar. At the distal end there is nearly a right angle between the surface 

 for the radius and that for the ulna. The ectepicondylar passage is a shallow groove close to 

 the radial border. 



