170 REPORT ON THE STATE CABINET. 



actually imperforate. I do not remember to have seen this feature in 

 any other variety or species of the genus. Those of the upper rocks, of 

 which separate valves are not uncommon, are always extremely thin at 

 this point ; the deltoidal space usually being open, from the loss of the 

 deltidial plates at an early period of growth. 



In the limestones of the Upper Helderberg group, at the Falls of the 

 Ohio, there occurs a form which is much larger than those of the Lower 

 Helderberg limestones. The ventral valve of this variety is flattened, 

 and usually a little concave toward the front, and the surface rather 

 coarsely striate, especially in the upper part of the shell. In this one, 

 so far as seen, the spiral ribbon is broad, the junction of the crura with 

 it abrupt, and at the posterior limit of the first volution ; the loop is 

 directed forward and reaches upward to some distance, but not so far as 

 in those from the Shaly limestones. The volutions of the spires appear 

 to be about fourteen or fifteen (Plate I, fig. 3). 



The variety occurring so abundantly in the soft shales of the Hamilton 

 group of New York, is similar to the last in form and size, the strise 

 rather finer. lu a very perfect individual of moderate size, the loop is 

 not so much elevated in the middle as in that one ; the volutions of the 

 spiral cones number twenty-two, and are very closely arranged ; the attach- 

 ment of the loop and principal band, nearly the same (Plate I, figs. 4 and 

 5). There is another variety found in I'ocks of this age at Independence, 

 Iowa, which is often of large size and extremely ventricose on the dorsal 

 side : the striae are fine, and closely arranged. In a specimen of this 

 variety, the volutions are only fifteen, and the upward curvature of the 

 loop very slight. 



In Atrypa spinosa. Hall, the loop curves upward but little, and extends 

 forward more than in A. reticularis from the same beds, while the volu- 

 tions of the spire are about fifteen. The principal distinction is in the 

 form of the spiral cones, and in the ribbon (Plate I, figs. 6 and 7). One 

 of the cones in the specimen used has been injured, which may have had 

 some influence in producing the modifications of the apex. The form of 

 the cone differs in having the upper part curved ; the apex being directed 

 backward, or towards the beak of the shell : the first basal volutions are 

 nearly straight on their inner sides, and the anterior extremity acutely 

 pointed, while the ribbon is abruptly widened near this part. 



I have been inclined to believe that the difference in the loop and 

 spires mentioned above may prove to be of some value in determining 

 whether these diffeiiBnt forms are really distinct species, or only changes 



