"Lower Silurian Fossils of Canada, 



207 



Fig 11. Fig. 



12. Fig 13. 



Fig. 14. 



Fig. 15. Fig. 16. 



Figs. 11, 12, 13, 14, 15, 16. — Different specimens of Atrypa increbescen^ 



Atrypa increbescens may be considered the cliaracteristic species of this 

 genus for the Trenton Limestone, as it occurs abundantly in the rock m 

 numerous and widely separated localities, shewing that it swarmed through- 

 out the sea, which, during the period when this formation was deposited? 

 covered all the middle portion of North America. This shell varies greatly 

 in form, according to the age of the specimen^ Tli€ young individuals are 

 somewhat oval or triangular, and flat with the mesial fold and sinus, little 

 developed. The old specimens are globular, and approaching the shape of 

 figures 15 and 16. The dorsal or shorter valve has a deep sinus, with three or 

 four ribs in the bottom, while the ventral valve has an elevated fold, with 

 four or five. The number of radiating ribs is usually fifteen. The beak of 

 the ventral valve is in the younger specimens acute and erect, slightly 

 incurved at the point, but in the old shells closely hooked over the apex of 

 the dorsal valve. The radiating ribs are never subdivided, but continuous 

 from the base to beak, becoming more slender as they proceed upward. — 

 They are crossed by imbricating zig-zag elevated lines, as seen in Figs. 15 

 and 16. This character ,^ however, is only seen in perfectly preserved shells. 

 Specimens are abundant, with the ribs well developed, but with little trace 

 of those lines. This species occurs in the Trenton Limestone. 



Atrypa plena is a fossil which occurs abundantly in certain layers of 

 the chazy limestone, and very closely resembles A. increbescens. The 

 number of plications or radiating ridges is, however, from 16 to 20, while 

 A. increbescens has, at the most only about 15. The ridges are also shar per, 

 and they are not crossed by the imbricating lines of the former species. 

 There are four or five ridges in the bottom of the ventral sinus, and five or 

 six on the mesial fold. We have seen some beds of limestone composed of 



