THALASSEMYDID&. 137 
“anterior rib-bearing marginal bone has been united with the middle disk by suture.” In the 
Vertebrata of the Cretaceous Formations of the West he says that the reference of the species 
to Catapleura is not final, ‘as the anterior costals and marginals are not known.” 
Of Cope’s type only a single bone was figured, the peripheral which he regarded as doubt- 
fully the tenth of the right side. In the Cope collection of fossil reptiles in the American 
Museum of Natural History this figured bone is found (plate 23, figs. 4, 5; text-fig. 152). It is 
accompanied by a number of other bones, some of which are markt in such a way that it is 
evident that Cope intended to figure them; besides other bones which probably do not belong 
to the species. It is evident that several of the bones belonging to the type have been fost, 
while it is probable that a few of other species have become mingled with it. It is certain, 
therefore, that our future identification of the species in new materials must depend princi- 
pally on the figured peripheral. The catalog number is 1131. Cope’s label shows that the 
type came from Barnesboro, New Jersey. 
It is evident that the bone in question does not belong to the right side. Usually in the 
related species the pit for the rib lies nearer the hinder end of the bone and the sulcus reaches the 
upper border in front of the pit; but if this bone is assigned to the right side, this pit will lie 
nearer the front end and the sulcus will fall behind the pit. The bone appears to be the seventh 
of the left side. It resembles closely the seventh of a specimen of O. borealis, but with specific 
differences. 
Cope has given the length and breadth of this bone as being respectively 2 inches g.3 lines 
(70 mm.) and 3 inches 2 lines (80 mm.), by length meaning evidently the distance from the 
suture with the bone in front to that with the bone behind, and by width the distance from the 
costal border to the free border. These dimensions will vary somewhat with the points selected 
152: , a 
oe 
153; 
154. 
Fics. 152-154.—Osteopygis chelydrinus. Sections of peripherals of type. X#. 
152. Section of seventh peripheral. 153. Section along intermarginal sulcus of second left peripheral. 
154. Section at front of third peripheral. 
on the borders. The free border of the bone originally measured at least 75 mm.; the 
upper border now measures 53 mm.; and the anterior border 80 mm. Cope states that 
these peripherals are remarkable for their shortness, but a careful comparison hardly confirms 
this statement. A more certain character appears to be found in the thickness of the bone. 
The thickness of the end regarded by the present writer as the anterior is slightly more than 
23 mm.; that of the other end is 19 mm. This is considerably more than the thickness of even 
the eighth peripheral of any known species, regard being had for the other dimensions. 
Another character has been noted by Cope, the angulation of the free border, at the end 
of the epidermal sulcus. The angle included between the two portions of this border is about 
130°. In other species the border is nearly straight or slightly concave where met by the 
sulcus. Usually in turtles, when the border of the carapace is notcht, the sulcus ends at the 
notch. Whether or not the angulation of the border is found in all of the posterior peripherals 
we have now no means of determining; but in all probability all were angulated. 
The pit for the extremity of the rib is elliptical in section and 29 mm. deep. The horizontal 
diameter is 15 mm.; the perpendicular, 11 mm. 
The bone may be regarded as having three faces, an upper, a lower, and an inner. The 
upper is slightly concave along a line from the costal to the free border. The epidermal 
sulcus is broad, but rather shallow. The sulci between the marginal scutes and the adjacent 
costal scutes appear to have run along close to the costal border. The lower face is consid- 
erably convex along a line from the free to the costal border. On this face there is a broad 
shallow sulcus, corresponding to the one above. The inner face contains the rib-pit. This 
face is concave and irregular, with the upper portion overhanging the lower. The bone did 
not form a sutural connection with the border of the first costal plate. 
