EXPLANATION OF THE PLATES AND WOOD-CUTS. 



plaits, its sides smooth ; lateral lobes reflexed, obscurely plaited. Length and 

 width nearly equal, the depth rather less. 



Somewhat resembling the most extravagant but small varieties of A. Pugnus 

 (M. C. t. 497.) ; but it is more triangular, the plaits are rounder, less elevated, con- 

 tinued almost to the beak, and more numerous ; it is more like T. flexistria (Ph. 

 loc. cit. PI. Xn. f. 33, 34.). 



A handsome shell. 



Loc. Plymouth. 

 Fig. 15. Atrypa Pugnus (M. C. t. 497.). A flattened variety of this very variable 

 species. See also Fig. 18. 



Loc. Plymouth. 

 Fig. 16. Atrypa protracta. Triangular, longer than wide, lateral angles rounded ; 

 beak produced acute ; sides flattened, front elevated with about 4 plaits, its sides 

 smooth. 



Loc. Plymouth. 

 Ftg. 17. Atrypa crenulata. Pentagonal, compressed, smooth ; front broadly and sud- 

 denly elevated, crenulated by numerous small plaits near its margin ; sides 

 smooth ; beak scarcely any. A neat species. 

 Fig. 18. Atrypa Pugnus (M. C. t. 497.). Another variety of the mountain-limestone 

 shell, rather narrower than usual ; other varieties also occur here. 



Loc. Mount Wise. 

 Figs. 19 and 20. Atrypa desquamata. Oblong, with a straight front, gibbous, deeply 

 striated ; striae increasing in number towards the margin ; front obtuse, gently and 

 broadly raised at the edge without elevating the surface ; shell longer than wide ; 

 upper valve deeper than the one with the projecting beak. 



This shell, which is very abundant in the Plymouth limestone, varies exceedingly 

 in size, coarseness of striae, and convexity. In some young individuals the form 

 is nearly globose, while in others it is compressed and lenticular. 



Loc. Plymouth. 

 Figs. 21 and 22. Atrypa desquamata var. compressa. Suborbicular, compressed ; sides 

 nearly rectangular ; margin nearly even ; valves equal. 



This differs from the first variety in being flatter and broader ; the upper valve, 

 not being very convex, does not hang over the beak of the lower so much as in 

 the more convex variety (Fig. 20.}, wherefore this seems to have a more projecting 

 beak, which, when broken, appears to have a flat area. 



Both the varieties approach so near to A. affinis (M. C. t. 324.), and more espe- 

 cially to the Eifel specimens of that species, that, were it not for the total absence 

 of concentric scales, the much more even margin, and the oval not triangular con- 

 tour, we should continue to refer them to the same species. One of the varieties 

 was formerly called T. affinis (in the Encyc. Metrop. loc. cit.). 

 Fig. 23. This is evidently a young shell ; we cannot distinguish it from some specimens 

 of ^. aspera (Silur. Syst. PI. XII. f. 5.), but most probably it is a young individual 

 of the A. desquamata. 



Loc. Plymouth. 



