168 REPORT ON THE STATE CABINET. ' 



spires, the loop curves gently foi'ward and upward ; the central or elevated 

 portion lying between and behind the cones, and forming a more or less 

 abrupt curve, or prolonged into a point directed towards the dorsal valve. 

 In some specimens where the origin of the loop is on the outer portion 

 of the first volution of the spire, the ribbon of the loop, and that of the 

 spire, are parallel for a short distance, that of the spire lying uppermost. 

 Where the origin is near the attachment to the hinge-plate, the looja has 

 the ajjpearance of being a continuation of the principal band ; while the 

 junction of the crura is more abrupt, they appearing only as supports of 

 the spires. 



The typical species of the genus (^A. reticularis) is known to exist in 

 most of the geological formations from the Clinton group to the Chemung 

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

 diflerent 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 diflerent authors as distinct 

 species, in consequence of their possessing features considered by them 

 to be of specific importance. At the present time, however, authors 

 most conversant with the subject agree in considering them only as 

 varieties of the typical form. 



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

 several diflerent varieties of Atri/jM reticularis, as well as in A. spinosa of 

 Hall, and I find that in the diflerent varieties of A. reticularis it is subject 

 to considerable variations of form. If, on further investigation, these 

 difierences should prove to remain constant in the several varieties, 

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

 connection with the diflerences in external features and perhaps some 

 modifications in the form of the spiral cones, serve as guides in establishing 

 specific characters in this group of shells, which has so long troubled 

 naturalists, and refused to conform to divisions founded on external 

 characters alone. 



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

 acters is often more injurious than beneficial ; but where species really 

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

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

 bringing to light some feature in this group of shells, which might serve 

 to determine more positively the relations these various forms, occurring 

 in the different geological deposits, bear to each other, that induced me 

 to continue the investigations, after discovering the existence of the loop 



