PLATE 23. 



Fig. 1. 453. 24. Atrypa congesta. (Pag- 67.) 



1 a. A young specimen. 1 6, c. Dorsal and fiont 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 c i; 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 fe, I. 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. 



1 n. A portion of the surface enlarged, showing the stria;. 



Fig. 2. 454. 25. Atrypa quadricostata. (P^g. 68.) 



2 a, 6. Ventral and front view of a young shell of this species. 

 2 c, d. Ventral and front view of an older individual. 



2 e. Profile view of the same. 2/. A portion of the surface enlarged, showing the striae. 



Fig. 3. 4-55. 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. 3 d. Profile view of the same. 



Fig. 4. 456. 27. Atrypa neglkcta. (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. .4trypa 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. 45S 29. Atrypa emacerata, (Pag. 71.) 



6 a, b. Two individuals of tliis species in green shale. 



6 c. The surface enlarged, shovving the concentric striaj. 



Fig. 7. 459. 30. Atrypa robusta. <! Pag. 71.) 



7 a, b. Dorsal and ventral valve of a full giown 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 6, c. Ventral and front view of a young specimen, in which the valves are equally convex, and the con- 

 centric laminse 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. Tlie 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, I. 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 7n, n. Front and profile view of a similar form, shov/ing the inequality of the valves and the mesial depression. 



Fig. 9. 461. 32. Atrypa plicatula. (Pag. 74.) 



9 a, b. 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 individnals. . 



9 A. A larger individual. 



Fig. 10. 462. 33. Atrypa hemispherica. (Pag. 74.) 



10 a, b. Dor,sal 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 c. A cast of the ventral valve. 



10 /. Several plications enlarged. 10 g. Profile of the elevation of the plications. 



Fig. 1). 463. 34. ^trypa planoconvexa. ( Pag. 75.) 



11 c. A young individual. 116. Ventral valve, with the plications nearly all simple, 

 lie. Dorsal valve of the s.ame 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. Tlie dorsal valve of another specimen, in which all the plications are dichotomous, numbering twenty- 

 six below the centre. 11 /. 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. 



