120 FOSSIL TURTLES OF NORTH AMERICA. 
The fragment of xiphiplastron present indicates a turtle of moderate size, but furnisht 
with a very thick and heavy plastron. It is estimated that the plastron had a total length of 
18 inches (about 460 mm.). The thickness, to the summit of the ischial articular surface, 
is 206 mm.; at the base of this, where it appears to be rising 
toward the pubic articular surface, 1g mm.; at the sutural 
border for union with the bone of the opposite side, 11 mm. 
In a specimen of Taphros phys molops, No. 1474, these measure- 
ments, in the order given, are 15 mm., 10 mm., and 10 mm. 
This specimen is estimated to have had a plastron 21 inches 
long. 
The lower surface of the bone is very flat and smooth. 
From the outer border the upper surface rises at first rapidly, 
then more slowly, to the base of the articular eminence, being 
thus convex. From the hinder border the upper surtace rises 
more and more steeply, being thus concave. The articular 
surface for the ischium is much larger than in Taphrosphys. 
It is approximately triangular in outline, the hinder border 
running somewhat parallel with the hinder border of the bone, 
On. while the outer border runs parallel with the outer border 
aie cea Gece of the bone and about 25 mm. distant from it. This articular 
$2. NO. 9078 MS surface extends mesially to the midline, differing thus from 
Taphrosphys and resembling Hydromedusa. The figure presented will show fhe form and 
extent of the surface. At its summit it has been comer eroded, and the border at the 
anterior angle may have been slightly extended. Where not eroded, the summit is roughened 
to afford attachment to the fechas At the anterior angle of the eminence the smooth Sce 
of the plastron at first descends, then begins to ascend, as af rising toward the articular eminence 
for the pubis; but no part of this is present. 
Fic. 133.—Natadochelys ingra- 
vata. Portion of left xiphi- 
Superfamily CRYPTODIRA Cope. 
Thecophorous turtles having the carapace composed of neurals, costals, and peripherals. 
The neurals in a few cases greatly reduced. The plastron having the epiplastra in contact with 
the hyoplastra. Entoplastron occasionally wanting. Mesoplastrals, so far as known, never 
present. Temporal roof varying from complete to obsolete. Pterygoids extending backward 
between the basisphenoid and the quadrates. Neck bending in a sigmoid curve in a vertical 
plane and capable of being retracted between the scapula. Elements of the pelvis never 
suturally joined to the carapace and the plastron. 
Of the Cryptodira there are 8 families and about 140 living species. Their geographical 
distribution is discust on page 31 and illustrated by fig. 10. To a greater extent than the mem- 
bers of any other superfamily he Cryptodira have been able to adapt themselves to the varying 
conditions of the globe. All lands, except the very coldest and the dryest, have been occupied 
by them. Most deserts have their species. Species inhabit forests and prairies, swamps, 
rivers, lakes, and the high seas. Their structure varies accordingly. By their wide geograph- 
ical distribution, the number of their species, and their great differentiation of structure, 
the Cryptodira proclaim themselves to be the successful group among the turtles. 
Geologically they reach back, according to present knowledge, to the Jurassic, being 
represented i in the Lower Kimeridgian of Europe by species of Thalassemy dide. 
Fig. 8 represents the author's present views regarding the origin and reJationships of the 
various families. Much remains to be determined as to these relationships. 
Family THALASSEMYDID Rutimeyer. 
Shell more or less incompletely ossified. Usually at least a central plastral fontanel 
present; often also costo-peripheral vacuities. Plastron loosely connected with the carapace; 
never closely sutured thereto. Limbs fitted for walking; never developt as flippers. Skull, 
when known, with the temporal fossze more or less caatt over. ‘Triturating surfaces of the 
jaws probably always broad. Neck short. 
