142 TWENTIETH ANNUAL REPORT OF THE REGENTS. 



group inclusiye, as well as in deposits of similar age in Europe. In the 

 diiFerenl formations it often presents peculiarities which are sometimes 

 quite characteristic of the beds in which they occur; so much so, that 

 several of them have been described by different authors as distinct species, 

 in consequence of their possessing features considered by them to be of 

 specific importance. At the present time, however, auth )rs most conver- 

 sant with the suliject agree in considering them only as varieties of the 

 typical form. 



1 have succeeded in ascertaining the existence and form of this loop in 

 several different varieties of Atrypa reticularis^ as well as in A. spino>a of 

 Hall, and I find that in the different varieties oi A. reticularis it is sub- 

 ject to considerable variations of form. If, o.i further investigation, these 

 differences should prove to remain constant in the several varieties, which 

 I am inclined to believe they will, they may, when considered in connec- 

 tion with the differences in external features and perhaps some modifica- 

 tions in the form of the spiral cones, serve as guides in ( stablishing specific 

 characters in this group of shells, which has so h-ng troubled naturalists, 

 and refused to conform to divisions ibunded on external characters alone. 



I am aware that the practice of founding species on insufficient charac- 

 ters is often more injurious than beneficial; but where species n^ally exist 

 in nature, it is well to know them, and to know the features which charac- 

 terize them, however obscure they may be. It was the hope of bringing 

 to light some featur^^ in this gi oup of shells, which might serve to deter- 

 mine more positively the relations these various form, occurring in the 

 difierent geological deposits, bear to each other, that induced me to con- 

 tinue the investigations, after discovering the existence of the loop connect- 

 ing the spires, more than two years ago. I had hoped to continue them 

 much further than I have yet done, previous to making known the results; 

 but circumstances have made it necessary to publish them at the present time. 



The differences described below, are some of the principal ones noticed 

 in the several varieties under which they are given. Of their importance, 

 I leave others to judge. 



The loop was first noticed in immature specimens from the Niagara 

 group, occurring at Waldron, Indiana. In the adult specimens from this 

 locality, the usual form of the shell is lenticular, with the dorsal valve a 

 little the most convex ; the surface not very finely ribbed, but very squa- 

 mose. The ribbon, or band forming the spiral cones, is wide ; the junction 

 of the loop with the principal ribbon is at a point distant from their 

 attachment to the hinge-plate, and not far from the point of greatest width 

 of the shell. The loop is strong and but moderately curved upward, with 

 a broad gentle curvature. The spire, in a specimen of moderate size, con- 

 sists of about twelve volutions ; the cones are erect, their apices a little 

 posterior to the centre of their bases. 



In the Shaly limestones of the Lower Helderberg group, we have a form 

 with a very ventricose dorsal valve, the ventral being only moderately so : 

 the surface is rather more finely ribbed than in the preceding variety, but 

 not so squamose. In this one the ribbon is slender, and the junction of 

 the loop much nearer to the hinge-plate ; while the loop is extended into a 

 long slender point, slightly recurved near its extremity, and reaches to 

 about half the hight of the spiral cones. The cones consist of about four- 

 teen volutions, and are situated very much as those of the Waldron 

 specimens are (Pl. I, fig. 1). 



Besides the difference of the external characters of this variety, as well 

 as of the loop and spires, there is a very noticeable peculiaiity in the beak 

 of the ventral valve. The entire beak, to the outer limits of the widely 



