18 FOSSIL TURTLES OF NORTH AMERICA. 
It is in the extensive group of Cry ptodira that we meet with the greatest number 
of what may be regarded as the minor modifications of the shell. If we except 
T oxochelys, no moan 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 i 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 
relate ely long and narrow to a breadth greater than the length. There may be 
one or more carinz 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 Foeanels 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 Echmatem ys, 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. ine G yclemys 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 j joint between the hy poplastron and the contig- 
uous peripherals. In Kinosternon there is a hinge between the epiplastra and the 
hy oplastra, 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 cece scutes, corresponding to the 12 rows of 
dermal hones of Derren: 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- 
temydidz. In the Emydide all have vanisht except one in the axillary notch and 
another in the inguinal. In the Baénidz, the Dermatemydide, and the Pleurodira 
we find the muddle plastral row represented by the intergulars. Archeochelys 
Lydekker and Polythorax Cope present more posterior scutes i this row. Between 
a young Dermochelys with its 12 rows of epidermal scutes, with many in each row, 
and Terrape ne, with only 7 rows, there are many and interesting stages. 
But the Trionychidze 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 
