PLATE 51. 



Page 

 Figs. 1-9. AtEYPA IMPRESSA 315 



1-3. Dorsal, ventral and front views of a specimen of tlie ordinary form. 

 4. 5. Profile and front views of a very gibbous form, liaving the mesial portion of the dorsal 

 valve more than usually inipresscd. 



6. A cast of a dorsal valve, showing the muscular impressions and the imprint of the crenulated 



socket plates. 



7. An enlargement from the upper portion of a specimen similar to that of figure 6, showing 



more distinctly the crenulations left by the removal of the socket plates. 



8. A ca-st of an extremely gibbous dorsal valve, with a very prominent muscular scar, showing 



vascular lines and papillose ovarian markings. 



9. C;ist of a ventral valve, showing muscular impression and dental cavities. 



Figs. 10-24. Atrypa reticularis , 316 



10. A dorsal valve of a broad form, with coarse plications, showing the marginal extension of 



the lamellsB ; from New York. 



11. A ventral valve without sinus, of the ordinary form of this species, as it occurs in the Cor- 



niferous limestone. 



12-13. Profile and front views of a specimen from Cayuga, C. W., having a sinus in the ven- 

 tral valve; of the ordinary size and form, as it occurs in the Coruiferous limestone of 

 New York and Canada. 



14-16. Dorsal, profile and ventral views of a large individual of this form, from New 

 York, the ventral side having the shell removed, showing the muscular markings and 

 the papillose surface of the interior. The specimen fig. 10, this and the following ones, 

 present some characters which might be regarded as of specific importance. The external 

 features, general form, striae, elevation of beak, etc., are distinctive, and may be found 

 useful in characterizing a form intermediate between those with fine striae, like repre- 

 sentations in figs. 11 -'13, and the .4. aspera or jl. spinosa. 



17. Dorsal view of a small specimen, from the hydraulic beds at the Falls of the Ohio. 



18 - 22. Dorsal, ventral, profile, cardinal and front views of a specimen illustrating the prevail- 

 ing form in the hydraulic beds at the Falls of the Ohio. 



23. Interior of a ventral valve, showing the muscular impressions and papillose surface. 



24. The hinge portion of the dorsal valve of the preceding specimen, showing the ba^es of 



crura and crenulate sockets; from the Falls of the Ohio. 



