PALEONTOLOGY. 

 PLATE VI. 



Page. 



Tancuedia nuLuosA — 370 



Fig. 1. View of a right valve enlarged, sLowing the prevailing form. 

 Fig. 2. A left valve slightly more extended at the posterior end. 

 Fig. 3. Outline cardinal view, showing the convexity of the valves. 



TaNCREDIA WAUltENANA 372 



Fig. 4. Enlarged view of a right valve, showing the features of the prevailing form. 



Tancuedia coubulii-ormis 370 



Fig. 5. Enlarged view of a right valve. 



Fig. (i. Enlarged view of a left valve. 



Fig. 7. Enlarged view of the right side of an internal cast, showing the muscular imprints. 



Fig. 8. Cardinal view of the name specimen. 



Tancuedia (f) ixornata 368 



Fig. 9. A right valve, natural size, showing the general features of the species. 



Fig. 10. View of an internal cast of a left valve, showing the form of the muscular imprints 



and a pallial sinus. 

 Fig. 11. View of a larger and proportionally longer left valve showing a slight diflfer- 



enco in the pallal sinus, and may, with the change in form, indicate a distinct 



species. 

 Fig. 1*2. Enlarged cardinal view of an internal cast, showing the impression of the cardinal 



and a lateral tooth, and the muscular scars. 

 I'lc. l:?. Enlarged view of another specimen preserving the shell along the hinge, andshow- 



ing the projecting cardinal tooth. 



Tancredia postica 371 



F"iG. 14. View of a right valve, enlarged, showing the great breadth of the posterior part of 

 the shell. 



HELEMXITES DEXSU8 .131 



Fig. 15. View of a young individual. 



Fig. 16. View of a large specimen broken off near the base of the alveolar cavity. 



Fkj. 17. Section of the upjx'r end of the specimen. Fig. 16, showing the excentricity of the 



alveolar cavity. 

 Fig. 18. Longitudinal section of a specimen, showing the concentric layers. 

 Fig. 19. Longitudinal section of another individual, showing the phragmacone in place, 



with some of the septa preserved. 



Ammonites cordiformis t 378 



Figs. 20, 21. Side and profile views of a moderately sized individual, showing the prevailing 



form. 

 Fig. 2*2. View of a small specimen from the Big Horn Mountains, in the collection of the 



Smithsonian Institution. 

 Fig. 23. View of another individual, showing a different arrangement of the costa. 

 Fig. 24. Diagram of a septum, copied from the Pal. Upp. Missouri, Plate v. Fig. 2 c. 



Ammonites cordifou.mis var. distans 380 



Fig. 25. Lateral view natural size of the specimen used in description. 



