l8 FOSSIL TURTLES OF NORTH AMERICA. 



It is in the extensive group of Cryptodira that we meet with the greatest number 

 of what may be regarded as the minor modifications of the shell. If we except 

 Toxochelys, no known Cryptodire shows more than vestiges of the ancient layer 

 of dermal bones. In some, as Protostega and Archelon, of the Upper Cretaceous, 

 the costal plates are nearly as much reduced as in Dermochelys. In these and a 

 number of other genera of Cretaceous turtles the peripherals are slender, but they 

 persist. Probably the neurals are never wholly absent. The elements of the 

 plastron degenerate in some cases; but, with the exception of the entoplastron of a 

 few genera, all persist. 



It is in this group that we discover the greatest variety in the forms of the shell, 

 ranging in convexity from much deprest to highly vaulted and bombous, and from 

 relatively long and narrow to a breadth greater than the length. There may be 

 one or more carinae on the carapace and its free borders may be smooth or variously 

 notcht or rolled. 



In some genera of Cryptodira, as the snappers and the sea-turtles, there are 

 extensive fontanels between the distal ends of the ribs and others at the sides and 

 the middle of the plastron. In most Cryptodira the bones are solidly articulated, 

 abrogating all fontanels. In the snappers and the sea-turtles again, the plastron 

 is only ligamentously joined to the carapace. In other genera, as Batagur, Hardella, 

 and Echmatemys, the plastron sends up powerful axillary and inguinal buttresses to 

 the inside of the carapace. Between these extremes there are all gradations. 



Among the Cryptodira there is a great variety of hinges between portions of the 

 shell. In Cydemys there is a hinge between the hyoplastron and hypoplastron, 

 and both these bones are sutured to peripherals. In Terrapene there is a similar 

 hinge, with the bones only ligamentously joined to the carapace. In the extinct 

 genus Ptychogaster there is a sliding joint between the hypoplastron and the contig- 

 uous peripherals. In Kinosternon there is a hinge between the epiplastra and the 

 hyoplastra, and another between the hypoplastra and the xiphiplastra. In Kinixys 

 there is a hinge in the carapace between the fourth and the fifth costal plates. 



Attention may be called to the modifications of the neurals and the costal plates 

 in Testudo. The neurals are alternately large and octagonal and small and quadrate. 

 The costals are truncated wedges, placed so that broad and narrow ends alternate 

 both next the neurals and the peripherals. 



There are numerous interesting modifications in the number, form, and disposi- 

 tion of the horny scutes of the shell among the turtles. There are supposed to have 

 been originally 12 rows, or zones, of these scutes, corresponding to the 12 rows of 

 dermal bones of Dermochelys and the ancestral turtle. A scute coincided with each 

 bone. In each row some scutes grew at the expense of the others and persisted even 

 after the disappearance of its supporting bone. Whole rows of the scutes dis- 

 appeared, as the supramarginals and the median plastral row of most turtles. The 

 supramarginals are represented in Macrochelys by a few scutes over the bridges. 

 The anterior and posterior ends of the supramarginal series are found in the Triassic 

 Proganochelys. Many genera furnish the inframarginals, as nearly all the Derma- 

 temydidae. In the Emydidae all have vanisht except one in the axillary notch and 

 another in the inguinal. In the Baenidae, the Dermatemydidae, and the Pleurodira 

 we find the middle plastral row represented by the intergulars. Archceochelys 

 Lydekker and Polythorax Cope present more posterior scutes of this row. Between 

 a young Dermochelys with its 12 rows of epidermal scutes, with many in each row, 

 and Terrapene, with only 7 rows, there are many and interesting stages. 



But the Trionychidae carry the reduction of the scutes to the extreme, for none of 

 the whole superfamily shows any traces of these whatever. In various species of the 



