EXPLANATION OF THE PLATES AND WOOD-CUTS. 



tudinally and finely striated ; striae interrupted by the concentric ribs ; hinge-line 

 equal to the width of the shell; beak somewhat produced. 



A small species, nearly akin to L. punctata (M. C. t. S2S). 



Loc. Plymouth. 

 Fig. 8. Atrypa juvenis. Broad-ovate, slightly convex, smooth, curved, longer than 

 wide ; front somewhat pointed ; valves nearly equal, the lower curved upwards 

 with a minute beak. 



A small, rather flat species, distinguished by its narrow front, and by being 

 curved, 



Loc. Plymouth. 

 Fig. 9. Atrypa lacryma. Subglobose, smooth, longer than wide ; front straight or 

 slightly waved, scarcely raised except at the edge, which is deeply sinuated by the 

 projection of the inferior valve, the central furrow of which is broad, flat and 

 bounded by two sharpish ridges ; its beak not prominent ; side rounded. 



An obscurely-marked species ; very convex, and with nearly equal valves. 



Loc. Plymouth. 

 Figs. 10, 11. Strigocephalus (Br onn) giganteus. (Terebratula gigantea,So-wevhy. See 

 Encyc. Metrop. loc. cit.) 



Suborbicular, convex, smooth, with a straight pointed beak ; valves nearly equal ; 

 area beneath the beak a right-angled triangle, flat ; foramen narrow. 



Almost all the specimens we have seen of this noble shell, which is often six 

 inches in diameter, are much distorted ; its true form appears to be round. It very 

 nearly resembles the genus Atrypa, but differs from it in the presence of an area 

 between the beaks ; this species is distinguished by its small straight beak, and its 

 flatter shape, from the Strigocephalus porrectus, which is plentiful in the Plymouth 

 limestone, and is {Ter. porrecta, M. C. t. 576. f. 1.) Strigoc. Burtini of continental 

 authors. 



A specimen with a slight depression along the lower valve we once distinguished 

 by the name of Ter. Hennahiana (see Encyc. Metrop. loc. cit.), but subsequent 

 examination has shown it to be a variety. 



In some internal casts the muscular impressions appear to have been strong, and 

 the shell has been coated inside with rhomboidal crystals of carbonate of lime, 

 which give it a decussated appearance. 



Loc. Plymouth, Newton Bushell. 

 Figs. 12, 13. Atrypa plebeia. Transversely obovate, with a produced front, smooth, not 

 very convex ; front slightly slevated, without a depression in the lower valve ; 

 beak but little prominent. 



This is a common species and varies in form ; it was formerly (see Ency. Metrop. 

 loc. cit.) considered to be the young of A ^/aira (M. C. Ji69. f. 1.), but the regular 

 convexity of the. lower valve and small beaks distinguish it. 



Loc. Mount Wise, Plymouth. 

 Fig. 14. Atrypa triloba. Tetrahedral, with rounded angles, 3-lobed, plaited ; upper 

 valve extremely gibbous ; lower nearly flat; front much elevated, with about 12 



