3i8 



GEOLOGICAL BIOLOGY. 



The species, like the genus, ranges from near the base of 

 the Upper Silurian to the Waverly or beginning of the Car- 

 boniferous age. "Almost coincident in time with the 

 appearance of Atrypa retiai/aris, in its typical aspect, we 

 find," writes Hall, " in the shales of the Niagara group shells 



Silurian 



Devonian 



Carboniferous 



.•.■■•.■Vf^A.'J^fcX'-'- 



'■•''■■■ii^*^'"'.: 



••.-.;Vw^^ii:v-.V 





Fig. gq. — A is a graphic expression of the nature of the differentiation supposed to have taken 

 place in the course of the history of the race, individuals of which are called Atrypa reticu- 

 laris. The lines and their relative position and length represent the divergence in varietal 

 modification and the continuance in generational repetition of like characters for the race. 

 S represents the groupings of the individuals at three successive stages of its history; viz., at 

 the beginning of the Silurian, near the beginning of the Devonian, and in the later part of 

 the Devonian era. The rather distinct specific grouping seen in the latter case is observed to 

 result from the dropping out of the inter. iiediate forms as well as by the increasing dominance 

 of the divergent forms. 



which are persistently small, with few and coarse plications, 

 more or less distinct median fold and sinus, and strong con- 

 centric lamellae. These shells have been designated as 

 Atrypa rugosa and A. nodostriata Hall" (p. 1 70); and these 



