PLATE 51. 
Page 
Figs. 1-9. Atrypa impressa.. . . 315 
1-3. Dorsal, ventral and front views of a specimen of the ordinary form. 
4, 5. Profile and front views of a very gibbous form, having the mesial portion of the dorsal 
•valve more than usually impressed. 
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 cast of an extremely gibbous dorsal valve, with a very prominent muscular scar, showing 
vascular lines and papillose ovarian markings. 
9. Cast of a ventral valve, showing muscular impression and dental cavities. 
Figs. 10-24. Atrypa reticularis... 816 
10. A dorsal valve of a broad form, with coarse plications, showing the marginal extension of 
the lamellae; 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 Corniferous 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 strife, like repre¬ 
sentations in figs. 11 -13, and the A. aspera or A. 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 bases of 
crura and crenulate sockets; from the Falls of the Ohio. 
