[26 



I'OSSI I I IK [ l.l-S OK NORTH AM K RICA. 



Fl< 



-Natadochelys ingra- 



vata. Portion of left xiphi- 

 plastron forming the type. 

 Xi No. 6078 A. M.N. II. 



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 26 mm.; at the base of this, where it appears to be rising 

 toward the pubic articular surface, 19 mm.; at the sutural 

 border for union with the bone of the opposite side, 11 mm. 

 In a specimen of Taphrosphys molops, No. 1474, these measure- 

 ments, in the order given, are 15 mm., 10 mm., and 10 mm. 

 I his specimen is estimated to have had a plastron 21 inches 

 long. 



The lower surface of the bone is very Hat 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 surface 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, 

 while the outer border runs parallel with the outer border 

 (it the bone and about 25 mm. distant from it. This articular 

 surface extends mesiallv to the midline, differing thus from 

 Taphrosphys and resembling Hydromedusa. The figure presented will show the form and 

 extent of the surface. At its summit it has been somewhat 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 ischium. At the anterior angle of the eminence the smooth surface 

 of the plastron at first descends, then begins to ascend, as if rising toward the articular eminence 

 for the pubis; bur no part of this is present. 



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 scapulae. 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 51 and illustrated by fig. 10. To a greater extent than the mem- 

 bers of any other superfamily the 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 in the Lower Kimeridgian of Europe by species of Thalassemydidae. 



Fig. 8 represents the author's present views regarding the origin and relationships of the 

 various families. Much remains to be determined as to these relationships. 



Family THALASSEMYDID-ffi Rutimeyer. 



Shell nunc 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 fossae more or less rooft over. Triturating surfaces of the 

 jaws probably always broad. Neck short. 



