PLATE 53 A. 



Pag« 



Pigs. 1 - 14, 18, 24, 25. Ateypa spinosa 822 



1. Dorsal view of a young individual, from tlie Cornifcrous limestone, showing the perforated 



beak, and the extended lainellie near the margin of the shell. 

 2-4. Dorsal, ventral and profile views of a specimen, of the ordinary form and size of thost^ 



<Vom the Upper Helderberg limestones of New York. 



6. Dorsal view of a large specimen from the Upper Helderberg limestone of New York. 

 ti. The interior of a ventral valve, from the Falls of the Ohio. 



7. The interior of an imperfect dorsal valve, from the Falls of the Ohio, showing the socket 



plates and muscular impressions; enlarged. 



8. 9. Views of a small and medium sized specimen, from the Hamilton shales, showing the 



spines. 

 10; 12. Dorsal, ventral and profile views of a specimen of ordinary size, showing the surface 

 denuded of the spines; from the Hamilton shales. 



13. Dorsal view of a large specimen without spines, from the Hamilton group. 



14. The interior of a ventral valve, showing mu.scular impressions, teeth, etc. 

 18. Dorsal view of a specimen, from the Chemung group, of New York. 



24. Dorsal side of a specimen, showing the spires, crura and loop. 



25. Ventral side of the same specimen showing the position of the spires, loop and attach- 



ment of the crura to the hinge-plate. From the collection of Mr. R. P. Whitfield. 



Pigs. 15-17. Atrypa HY8TRIX 326 



15. Ventral view of a small specimen which preserves the marginal spines. 



16. View of an individual preserving several spines toa length of nearly three-fourtlis of an inch. 



17. View of an individual having very strong symmetrical i>lications. 



Figs. 19-21. Atrypa aspera var. occidentalis 



19-21. Veutral, profile and dorsal view of a specimen, showing the prevailing form of this si)c- 

 cies as it occurs at Inde)>cndence, Iowa. 



Pigs. 22, 23. Atrypa reticularis 316 



22. The vential side of a specimen, sliowing the position of the spiral cones, with their attach- 



ment to the hinge-plate, and tlie loop connecting tike spires. 



23. Dorsal side of the same specimen. Fr<mi the collection of Mr. K. P. Whitfield. 



