208 



Loicer Sihiriaii Fossils of Canada^ 



a dense mass of these fossils, packed closely together and well preserved. 

 Plenus, Latin, full, large or plentiful. 



Fig. 18. 



Fig. 19. 



Fig. 21. Figs. 22 and 23. 



Figs. 17, 18, and 19. — Atrijpa plena. 



Figs. 20, 21, 22, and 23.— Atrypa hemipUcata. 



Airy pa hemipUcata is easily recognised by its smoothness in the upper 

 part, and by the broad folds below, which extend only half way to the 

 beaks. This fossil is somewhat pentagonal, wider than long, and generally 

 rather flat than globose. The sinus as well as the broad plications extend 

 only half the length, and the surface is sometimes marked " by fine, 

 concentric, filiform, subimbricating lines, which are more conspicuous 

 towards the base of the shell, and beautifully undulated in crossing the 

 plications." The beaks are very short, and about equally incurved. It is 

 rather common in the Trenton limestone, but not so abundant as J. 

 increbescens. The specific means simply half-plicated. 



Professor Hall gays : — " This peculiar and beautiful species is rarely 

 found in western localities, but is nevertheless extensively distributed in 

 Xew Tork. It is readily distinguished by its usually rotund figure, and 

 short prominent plications in the lower half of the shell, while it is smooth 

 above, or only marked by fine concentric lines. The sinus at the base 

 usually occupies one fifth of the cu-cumference of the shell, which, with the 

 basal and cardinal slopes, give it a pentagonal appearance. In many 

 specimens collected from the crystalline limestone, I have been able to 

 observe only the plications on the mesial portion, while in the softer or shaly 

 limestone, the lateral ones have all the distinctness of those presented in the 



tigureSi" 



Palaeontology of New York, Vol. 1, Page 144. 



