114 



A POP17LAR EXPOSITION OF THE 



Atrypa .—A good deal of uncertainty still prevails ^^^th regard to 

 tte proper limitation of this genus. In outward form it agrees with 

 Bhynconella, see below, but appears to 

 possess internal calcareous spires, the 

 points of which extend into the hollow of 

 the smaller or dorsal valve. Fig. 102 

 represents an exceedingly common spe- 

 cies, A, reticularis, of the Upper SUu- 

 rian and Devonian strata, but chiefly 

 characteristic of the latter. Pig. 102, 



Bhynconella : — Shell, in general, strongly bi-couvex. Hinge-line, 

 curved ; no area. No internal spires, but in the living species the 

 arms are coiled spirally, the spires pointing downwards and inwards. 

 The genus ranges from the Lower Silurian into the existing epoch. 

 Fig. 103 represents a small form, B. plena, very common in the Chazy 

 limestone of the Trenton Group, (Lower Silurian) ; and fig. 105, B. 

 increbescens, a closely related species occurring abundantly through- 

 out the Trenton limestone. In this latter species, the plications 



Pis. 103. 



Fig. 104. 



Fig. 105. 



on the shell are crossed by well-marked imbricating lines of growth. 

 Numerous examples of this genus occur also in our Upper Silurian 

 and Devonian strata. A modern species, found in the Post -Tertiary 

 deposits of Eastern Canada, R. psittacea, is figured in the wood- 

 cut i05. 



Fig. 106 is a representation of the old Bhyn- 

 conella hemiplicata of the Trenton Grroup, now 

 referred by Mr. Billings to his new genus Came- 

 rella. It is characterized by a few broad plications 

 Fi" 106 ^^ ^^ lower part of the shell. 



