PLATE 23. 
Fig. 1. 453. 24. Atrypa congesta. (Pag- 67.) 
1 a. A young specimen. 1 b, c. Dorsal and front view of a young specimen. 
1 d, e,f. Ventral view of three different specimens, showing the progressive development in the lateral folds 
of the ventral valve. 
1 g, h. Front view of e fy f, showing the deep sinus in the ventral valve, and the linguiform process of the 
dorsal valve. 1 i. Profile view of a full grown specimen. 
1 k, l. Dorsal valve and cardinal view of a specimen, where the lateral folds are more than usually developed. 
1 m. Interior of the dorsal valve, showing the simple dental processes on each side of the foramen. 
In. A portion of the surface enlarged, showing the striae. 
Fig. 2. 454. 25. Atrypa quadricostata. (Pag. 68.) 
2 a, b. Ventral and front view of a young shell of this species. 
2 c, d. Ventral and front view of an older individual. 
2 c. Profile view of the same. 2/. A portion of the surface enlarged, showing the striae. 
Fig. 3. 455. 26. Atrypa bidens. ( Pag. 69.) 
3 a, b. Ventral valve of two specimens of different age. 
3 c. Front or base of the larger specimen, showing the elevation of the sinus. 3d. Profile view of the same. 
Fig. 4. 456. 27. Atrypa neglecta. (Pag. 70.) 
4 a. A specimen with seven plications on each side of the mesial fold. 
4 b, c. Profile and front views of the same specimen. 
4 d, e, f. Ventral, dorsal, and profile view of a specimen with only five plications on each side of the mesial 
fold. 
Fig. 5. 457. 28. Atrypa equiradiata. (Pag. 70.) 
5 a, b. Ventral and profile view of a small specimen. 
5 c, d, e. Dorsal valves of three individuals of different size. 
Fig. 6. 458 29. Atrypa emacerata. (Pag. 71.) 
6 a, b. Two individuals of this species in green shale. 
6 c. The surface enlarged, showing the concentric stria:. 
Fig. 7. 459. 30. Atrypa robustA. (Pas'- 71.)' 
7 a, b. Dorsal and ventral valve of a full grown specimen. 7 c. Profile view of the same. 
7 d. Front view of the same, showing the three plications in the mesial sinus, and four on the elevation. 
Fig. 8. 460. 31. Atrypa reticularis. ( Pag. 72.) 
8 a. A specimen of the ordinary size of this species in the Clinton group. 
8 b, c. Ventral and front view of a young specimen, in which the valves are equally convex, and the con¬ 
centric laminae very rugose. 
8 d, e. Dorsal valve and profile view of an older specimen, with the valves almost equally convex. 
8 f, g. Ventral valve and cardinal view of a specimen slightly larger than the preceding, in which the dorsal 
valve is more convex, with a slight depression in the ventral valve, reaching from the beak halfway to 
the base. 
8 h. The dorsal valve of a specimen where the shell is partially exfoliated. 
8 i. The cast of the dorsal valve of a similar form, where the plications are flat and undivided. 
8 k, l. The ventral and dorsal valves of a young specimen, where the ventral valve is flatter than the dorsal, 
and has a distinct mesial depression. 
8 in, n. Front and profile view of a similar form, showing the inequality of the valves and the mesial depression. 
Fig. 9. 461. 32. Atrypa plicatula. ( Pag- 74.) 
9 a, 6. Ventral valve and profile view of a young specimen. 
9 c. Profile view of another specimen, in which the valves are still quite unequal. 
9 d, e. Front view of a young shell and a full grown individual. 
9 /, g. Dorsal and ventral valves of a specimen having the ordinary characters of full grown individuals. 
9 h. A larger individual. 
Fig. 10. 462. 33. Atrypa hemispherica. (Pag. 74.). 
10 a, b. Dorsal and ventral valve of young specimens. 
10 c. A large dorsal valve, showing the concentric striae. 
10 d. A group of individuals on a-fragment of stone. 10 e. A cast of the ventral valve. 
10 /. Several plications enlarged. 10 g. Profile of the elevation of the plications. 
Fig. 11. 463. 34. Atrypa planoconvexa. (Pag- 75.) 
11a. A young individual. 116. Ventral valve, with the plications nearly all simple. 
11 c. Dorsal valve of the same specimen, showing the plications more dichotomous. 
11 d. The dorsal valve, having a few of the plications in the middle larger than the others, and a small inter¬ 
stitial one in the centre. 
11 e. The dorsal valve of another specimen, in which all the plications are dichotomous, numbering twenty- 
six below the centre. 11 f. Profile view of a specimen of ordinary size. 
11 g. A cast of the ventral valve. 11 h. The interior of the hinge of a ventral valve. 
11 i. Profile of the elevation of the plications. 
