PI.ASTOMENID^E. 467 



especially struck by the configuration of" the inner end of the hyoplastral bone, which appeared 

 to him greatly different from that of Trionyx; but he does not mention the condition of the 

 median and hinder portions of the plastron. 



In 1877 (Wheeler's Surv. looth Merid., iv, p. 47), Cope concluded that the hyoplastrals 

 were not greatly different from those of Trionyx, while the hypoplastrals were materially 

 different. They are stated to have displayed a transverse width behind the inguinal region 

 more like an emydoid than a trionychoid genus. The more extended ossification of all these 

 bones in Plastomenus than in other genera of trionychids was recognized. Cope's remarks 

 on the genus in the work referred to were copied into his work of 1884 on the vertebrates of the 

 Tertiary formations. 



Even as late as the year 1882 (Amer. Naturalist, xvi, 991) Professor Cope was uncertain 

 whether or not this genus belonged to the Emydidae. 



Several of the species referred to this genus are based on very fragmentary and incomplete 

 materials, and it is not improbable that when more complete remains shall have been dis- 

 covered some of these species will have to be placed in other genera. 



ANALYSIS OF SPECIES REFERRED TO PLASTOMENUS. 



A 1 . Judith River and Laramie species: 



1. Costals with angular pits, 5 in 10 mm., and crost by prominent welts costatus 



A*. Arapahoe species: 



2. Costals without welts; pits fine, not angular, 6 in 10 mm punctulatus 



3. Costals not known; free border of hypoplastron acute; lower surface with 



angular pits insignis 



A 3 . Puerco or Torrejon species: 



4. Carapace with welts. Carapace and plastron with small pits acupictus 



A*. Wasatch species: 



5. Like P. thomasi, but ridges between pits broader; 7 pits in a line 15 mm. long catenatus 



6. Costals thick; pits on distal ends of costals in rows; 3 or 4 pits in a line of 10 mm. leptomitus 



7. Costals thick; sculptured with pits and welts; pits circular, 7 on 10 mm lachrymalis 



8. Costals of moderate thickness, pits angular, 7 in 10 mm; some of the intervening 



ridges weltlike fractus 



9. Pits of costals abruptly sunken; 5 in 10 mm. ; no welts; plastron with ver- 



miculate ridges corrugatus 



10. Costals thick; pits circular, 4 in 10 mm.; oblique welts on some costals; plastron 



with rows of circular pits communis 



A*. Bridget species: 



11. Carapace with welts and pits; four pits in a 10 mm. line. Costals of eighth 



pair as long as wide thomasi 



12. Carapace with welts and pits; five or six pits in lomm.; eighth costals much 



shorter than broad visendus 



13. Carapace with welts and pits; eight pits in 10 mm.; hinder costals about as 



long as wide tantillus 



14. Carapace with welts on hinder end; three or four pits in 10 mm trdemius 



15. Like oedemius but with from five to seven pits in 10 mm molopinus 



Plastomenus costatus Cope. 

 Plate 85, figs, i, 2. 



Plastomenus costatus, COPE, Vert. Cret. Form. West, 1875, pp. 94, 261, plate viii, fig. 8; Proc. Acad. 

 Nat. Sci. Phila. 1875, P- 9i Brit. N. A., Bound. Comm. Report on Geol. etc., 1875, p. 337. HAY, 

 Bibliog. and Cat. Foss. Vert. N. A., 1902, p. 452. HATCHER, Bull. U. S. Geol. Surv. No. 257, p. 74. 



This species was described from materials collected by Dr. G. M. Dawson, south of Wood 

 Mountain, Assiniboia. The deposits from which they were derived were supposed to belong 

 to the Laramie, but are now known to belong to the Judith River. These materials are now 

 the property of the Geological Survey of Canada. They consist of a piece of a costal, a frag- 

 ment of probably the right hypoplastron, and another small undetermined fragment. The 

 costal fragment was figured by Cope as cited. The bone is remarkable for the relative thick- 



