TRIONYCHIDJE. 



The living species included by the present writer under this genus are those embraced 

 under the heads I, A and I, B, 2 and 3, of Dr. Boulenger's synopsis of species, on page 244 of 

 his Catalogue of Chelonians. The type of the genus is Trionyx euphraticus of the same work. 

 It is possible that Trionyx subplanus Geoffr. ought to be regarded as the type of the genus 

 Dogania. Besides the fossil North American species whose remains are complete enough to 

 prove that they belong under the present genus, a considerable number of others are placed 

 here provisionally. 



For a discussion of the name Amyda see Stejneger, Science, xxi, 1905, page 228. 



As remarkt under the genus Aspideretes it seems most probable that the upper Cretaceous 

 species placed here under Amyda really belong under Aspideretes. 



KEY TO NORTH AMERICAN SPECIES OF AMYDA. 



A 1 . Upper Cretaceous species: 



1. Sculpture coarse, mostly of circular pits, about 4 in line of 16 mm prisca 



2. Sculpture still coarser; 3 pits in line 15 mm. long; pits deep, rising gradually to 



summit of wall halophila 



A*. Species of Atlantic slope Eocene: 



1. Middle of costal with irregular pits, the sutural borders with ridges somewhat 



oblique to the borders; about 2 ridges in a 10 mm. line pennata 



2. Costals much thicker than in pennata; pits and ridges coarse as in pennata 



but irregular or in rows at right angles with the sutural borders virgtntana 



A 3 . Wasatch species: 



1. Costals thick and growing thicker toward outer end; sculpture coarse; irregu- 



larly arranged and deeply imprest pits; 4 or 5 in line 20 mm. long canosa 



2. Costals thick; sculpture mostly of winding and anastomosing ridges inclosing 



pits and short furrows; 2 or 3 ridges crost by line of 5 mm ventricosa 



3. Costal bones thinner than in cariosa; the sculpture similar radula 



A*. Bridger species: 



1. Carapace nearly as wide as long; front of nuchal long and nearly straight; with 



a smooth band in front; 5 or 6 pits in 20 mm aqua 



2. Carapace nearly as broad as long; 4 to 7 pits, usually 5, in 20 mm. line; no 



depression along back; nuchal, from side to side, 0.64 the width of the shell .... utntaensts 



3. Carapace nearly as wide as long; a conspicuous depression along the back; length 



of nuchal 0.80 of width of shell; with a smooth band in front scutumantiquutn 



4. Costal bones thin; on proximal ends of costals, 2 pits in 10 mm.; on distal ends, 



3 or 4; some ridges crossing the costals more conspicuous than others concentnca 



5. Carapace nearly as broad as long; nuchal not half as wide as carapace; sculp- 



ture of median coarseness, 3 pits in 10 mm franctscie 



6. Carapace broader than long; nuchal nearly 0.6 the width of the carapace; 



sculpture coarse and irregular, on central portions from 2 to 5 pits in 20 mm . . salebrosa 



7. Carapace about as broad as long; the bones thin; nuchal concave in front, its 



length nearly 0.7 the width of the carapace; a smooth band along its front; 



sculpture coarse, about 2 pits in 10 mm. line exquisita 



8. Carapace nearly as broad as long; rounded in front and behind; nuchal a little 



more than half the width of the carapace; on proximal ends of costals, 3 pits 



in 15 mm., on neurals and on distal ends of costals 5 or 6 pits in 15 mm. line. . mtra 



9. Known only from skull, which resembles that of A. ferox; crushing surfaces of 



upper jaw broad tritor 



10. Carapace nearly as broad as long; nuchal not one-half the width of the carapace; 



sculpture coarse, 3 or 4 pits in 20 mm. line egrtgia 



A*. Uinta species: 



I. Carapace broadly oval; nuchal 0.65 the width of the carapace; sculpture coarse, 



1 or 2 pits in 10 mm. line crassa 



A*. Miocene species: 



1. Costals thick and heavy; crost by narrow, sharp ridges, like those of a file, 



2 in about 7 mm lima 



2. Costals not so thick; ridges closer together than \nlttna and curving across costal butet 



3. Pits of costals closely placed, irregular in size and remarkably deep cellulosa 



