S14 PALEONTOLOGY OF NEW YORK. 



The muscular area in the bottom of the dorsal valve is usually divided 

 by a median ridge; and in old shells of some of the species, this area 

 becomes very strongly defined. 



The vascular impressions, in both valves, originate on each side near the 

 base of the muscular area, and divide into two principal trunks which 

 diverge towards the hinge-line and the front of the shell, and give off 

 numerous lat-eral bifurcating branches. 



The prevailing and conspicuous forms of this genus have been generally referred 

 to two species, or rather to a single species, the Atrypa reticulana of Linnaeus and 

 its varieties-; though more recently the A. desquamata of Sowerby has been 

 doubtfully admitted as a dfstinct species. My own investigations do not lead to 

 the adoption of this view, though I admit that there are many difficulties in the 

 way of specific detcnnination, especially where the shells are exfoliated or com- 

 pressed ; but the examination of large numbers of individuals, together with the 

 interiors of many of them, hiis compelled me to recognize as distinct species some 

 of those which are usually considered as varieties only. How far the complete 

 study of the interior structure, and of the spires, will sustain the separation of 

 species, remains yet to be determined. 



In order for a satisfactory solution of all these difficulties, a series of observa- 

 tions should be made upon extensive collections of individuals from widely separated 

 localities in the same geological horizon, and also from different geological forma- 

 tions, to learn the effect of geographical and chronological influences. I conceive 

 that until such investigations shall have been made, we are not prepared for a 

 final determination of the questions of specific difference or identity. 



Two species of this.genus are of common occurrence in the Niagara and one in 

 the Lower Helderberg group ; and though I have referred the more common form 

 to A. reticularis, there is usually no difficulty in recognizing the specimens from 

 each formation, or in distinguishing them from those of the Upper Helderberg 

 and Hamilton groups. The same observations are almost equally true of those in 

 the higher rocks ; but at the same time we find strongly marked features of dis- 

 tinction in individuals from widely separated localities of the same formation. 



