510 FOSSIL TURTLES OF NORTH AMERICA. 
close to the midline. Seventy-five millimeters of the front border of the bone is present. 
The greatest thickness of the bone is 12 mm., but most of it is thinner. The posterior half of 
the upper surface is ornamented with a system of well-defined and rather sharp ridges which 
inclose pits. Of these there are 3 or 4in 10 mm. The anterior half of the surface was devoid 
of ornamentation, but is markt by numerous grooves and perforations for blood-vessels. The 
connection of the nuchal with the first neural and the first costal was apparently like that 
found in T. puercensis. There remains on the posterior border of the nuchal about 20 mm. of 
the dentated sutural edge for union with the first neural. Distad of this the border is smooth 
for a distance of 30 mm.; so that there have evidently existed between the nuchal and the first 
pair of costals two elongated fontanels, separated by the anterior neural. At the distal end of 
each fontanel the border turned backward, to come into contact with the costal. What the 
breadth antero-posteriorly of this fontanel was, we can not say, but since there appears to have 
been only a single neural between the first pair of costals, the fontanel was probably narrow. 
The greater part of the first costal is present, only the distal extremity being gone. The 
average thickness is about 8 mm. The greatest fore-and-aft extent is 57 mm.; the lateral 
extent is a little over 100 mm., the thickness, 6 mm. The bone was originally about 125 mm. 
long. The anterior border for about 35 mm. proximally did not articulate with the nuchal, 
ue formed the posterior border of the fontanel already mentioned. Distad of this, the front 
edge of the bone was deeply grooved for the hinder edge of the nuchal. The proximal end of 
en aa 
hypo 
Fics. 668 AND 669.—A xestemys byssina. 
ub 
4° 
668. Plastron. Xiphiplastron of type; hypoplastron from 
No. 1034 A. M. N. H. 
669. Part of nuchal and left first costal. 
the bone articulated with the first neural. The direction of the proximal border shows that 
the anterior neural was broader at the anterior end than behind. The hinder border of the 
costal is jagged, for sutural union with the next costal. The rib-end of the first costal plate 
seems to have been overlapt by the outer end of the nuchal. 
This species was a large one. The free borders of the costals were beveled off very ob- 
liquely to a sharp edge and on this beveled surface decussating bony fibers, like those of the 
plastron, are seen in abundance. How far the free portion or the ribs extended beyond the 
border of the carapace can not be determined, but the distance must have been considerable. 
The sculpture of the costals resembled that of the nuchal; but, in places at least, the pits 
are larger and the ridges sharper. The walls slope away gradually toward the bottom of the 
pits. There is little or no tendency toward the arrangement of the pits in rows. 
There appears to be sufficient reason for believing that the plastral and the carapacial 
bones here described belonged to the same individual. 
Genus AMYDA Oken. 
Carapace normally with 8 pairs of costal plates. No preneural present. One or more of 
the posterior pairs of costals may be in contact on the midline, or all may be separated by the 
neurals. Young in living forms with numerous longitudinal dorsal dermal ridges or series of 
tubercles. 
Type: Trionyx euphraticus Daudin. 
