t'HELONIID^. 221 



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 is smooth, showing that there were costo-penpheral 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 

 90 . 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 thev soon enter the bone. In the area of the vertebral scute the surface is pitted bv vascular 

 openings. The anterior border of the bone is grooved somewhat at right angles with the 

 sutural edge. 



Family CHELYDRID^ 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 costitorm 

 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 Devisia of New Guinea. The first contains two species ; Macrochelys 

 and Devisia onlv 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 murchtsoni is found in the Upper Miocene of Switzer- 

 land and C. deeheni in the Upper Oligocene of the Rhine region, near Bonn. 



While the Chelydridae are in many respects primitive in their structure, in other respects 

 thev have deviated considerably from their Amphichelvdian ancestors. The roof of the tem- 

 poral region has suffered considerable reduction. The plastron has undergone reduction. 

 The writer can not believe that the opisthocoelous 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 Chelydridae are related to the Chelo- 

 niidae through some Cretaceous ancestor of Toxochelys. Altho he has indicated (Bull. Amer. 

 Mus. Nat. Hist., xxi, 1905, p. 167) that the Emydidae and the Testudinidae sprang from the 

 Chelydridae he is now inclined to hold that thev had their origin from the Dermatemydidae. 



Genus ACHERONTEMYS Hay. 



A genus of Chelydridae. Carapace broad, considerably deprest. Neural bones eight, 

 about as broad as long, mostly hexagonal, with the broader end forward. A single broad 

 suprapygal. Peripherals, n pairs, placed opposite the ends of their respective costals. Verte- 

 bral scutes very broad. 



Type: Acherontemys heckmam Hav. 



The relationships of the present genus are yet somewhat uncertain, but they are believed 

 to be with the Chelydridae. A comparison of the plan of the carapace of the species described 

 below v. itli 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 Emvdidae. The peripherals do not alternate with the costals. 

 Acherontemys differs from the living genera mentioned in having the costals more closelv 

 articulated with the peripherals and in possessing extremely broad vertebral scutes. 



