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 spetiiuen. 



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 {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 fc, I. Dorsiil 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 simjile dental processes on each side of the foramen. 



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



Fig. 2. 4.54. 25. Atrypa QnAuRicosTAXA. (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 e. Profile view of the s;ime. 2/. A portion of the surface enlarged, showing the stria-. 



Fig. 3. 4-55. 26. Atrypa bidens. ( Pag. 69.) 



3 a, b. Ventral valve of two specimens of ditlerent 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 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, dorsivl, and profile view of a s{>eoinien with onlv five plications on each side of the mesial 



fold. 



Fig. 5. 457. 2S. Atrypa f.quirauiata. (Pag. 70.) 



5 a, b. Ventral and profile view of a small specimen. 



•1 c, d, e. Dorsal valves of three individuals of ditt'erent size. 



Fig. 6. 458 29. Atrypa emacerata. (Pag. 71.) 



6 o, 6. Two individuals of this species in green shale. 



6 c. The surface enlarged, showing the concentric stria;. 



Fig. 7. 459. 30. Atrypa robusta. (Pag- 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 s;ime, showing the three plications in the mesial sinus, and four on the elevation. 



Fig. 8. 460. 31. Atrypa RETicirLARis. ( 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 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 sliglitly 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, 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 m, 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, b. Ventral valve and profile view of a young specimen. 



9 c. Profile view of another specimen, in which the valves arc still quite unequal. 



9 d, e. Front view of a young shell and a full grown individual. 



9 /", g. Diir.sal 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 specuuens. 

 10 c. A large dorsal valve, showing the concentric striae. 



10 d. A grtmp 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. 11. 463. 34. Atrypa planoconvexa. ( Pag. 75.) 



11 (I. A young individual. 11 b. Ventral valve, with the plications nearly all simple. 

 11 c. Dorsal valve of the same specimen, showing the plications mc^re dicliotoraons. 



11 d. The ilorsal 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 arc dichotomous, numbering twenty- 

 six below the centre. 11 f. Profile view of a spccimCTi 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. 



