CHELONIIDE. 22k 
The costal here described (plate 32, fig. 3) appears to have been the sixth of the right side, 
and to have belonged to an individual whose carapace had a length of about 400 mm. The 
length of the bone, from the neural border to the distal end, is 125 mm. The width at the 
middle of the length is 44 mm. The thickness at the sutural border is 5 mm. The rib adds 
about 3 mm. through the middle of the width. The distal end of the bone is cut off obliquely 
and the margin 1s smooth, showing that there were costo-peripheral fontanels and no suture 
with the peripheral. 
On the proximal end of the upper surface appears a portion of a vertebral scute, probably 
the fourth. The outer angle is distant 45 mm. from the neural border and the angle is less than 
= : : 
go°. The sulcus between the third and the fourth costal scutes runs on the bone near the 
posterior border. 
The upper surface of the bone is markt by numerous vascular grooves. In general, these 
run toward the distal end, branching and anastamosing. When followed toward the proximal 
end they soon enter the bone. In the area of the vertebral scute the surface is pitted by vascular 
openings. The anterior border of the bone is grooved somewhat at right angles with the 
sutural edge. 
Family CHELYDRIDZ Agassiz. 
Plastron loosely joined to the carapace; consisting of nine elements and considerably 
reduced. The entoplastron T-shaped; the bridge narrow. Nuchal bone with long costiform 
processes. A full series of neurals. Skull with temporal region incompletely rooft over; the 
postfrontal bones large. Quadrate notcht for passage of the stapedial rod. Crushing-surfaces 
without ridges and processes. Caudal vertebra mostly opisthoccelous. 
At the present day this family is represented by 3 genera, Chelydra and Macrochelys of 
North America and Devista of New Guinea. The first contains two species; Macrochelys 
and Devisia only one each. Chelydra serpentina is the best-known species and ranges east 
of the Rocky Mountains from Canada to Ecuador. 
The members of this family are regarded as being among the most primitive of the living 
turtles. Nevertheless, known remains of the family have not been found in deposits earlier 
than the Upper Oligocene. Chelydra murchisoni is found in the Upper Miocene of Switzer- 
land and C. decheni in the Upper Oligocene of the Rhine region, near Bonn. 
While the Chelydride are in many respects primitive in their structure, in other respects 
they have deviated considerably from their Amphichelydian ancestors. The roof of the tem- 
poral region has suffered considerable reduction. The plastron has undergone reduction. 
The writer can not believe that the opisthoccelous caudal vertebra are primitive. The costi- 
form processes of the nuchal are a late acquisition. 
As the writer has stated elsewhere, he holds that the Chelydrida are related to the Chelo- 
niide through some Cretaceous ancestor of Toxochelys. Altho he has indicated (Bull. Amer. 
Mus. Nat. Hist., xx1, 1905, p. 167) that the Emydide and the Testudinide sprang from the 
Chelydride he is now inclined to hold that they had their origin from the Dermatemydide. 
Genus ACHERONTEMYS Hay. 
A genus of Chelydride. Carapace broad, considerably deprest. Neural bones eight, 
about as broad as long, mostly hexagonal, with the broader end forward. A single broad 
suprapygal. Peripherals, 11 pairs, placed opposite the ends of their respective costals. Verte- 
bral scutes very broad. 
Type: A cherontemys heckmant Hay. 
The relationships of the present genus are yet somewhat uncertain, but they are believed 
to be with the Chelydrida. A comparison of the plan of the carapace of the species described 
below with those of the carapaces of Chelydra and Macrochelys (Boulenger’s Cat. Chelonians, 
pp- 22, 24) will show that there are great resemblances. The nuchal bone has not the back- 
ward extension seen in the Emydide. The peripherals do not alternate with the costals. 
Acherontemys differs from the living genera mentioned in having the costals more closely 
articulated with the peripherals and in possessing extremely broad vertebral scutes. 
