INVERTEBRATES. 309 



12 inches; dorso-ventral diameter of inner whorl, 8 inches; 

 breadth of umbilicus, 8.50 inches ; circumference around the 

 periphery, 4 feet, 8 inches. 



The specimen from which our figures of this species were drawn, natural size. 

 we now know to be a young shell, or merely the inner whorls of an adult. Owing 

 to the fact that its last septum is broken on the inner side, the lobe is not visi- 

 ble, and it was only after recently breaking one of the volutions across at another 

 septum, that we discovered its existence in this specimen. Hence on the dis- 

 covery of the great ponderous shell in which this lobe was first discovered, and 

 from which the foregoing description and measurements are given, its specific 

 identity with that figured on plate 25 was not suspected, and another name was 

 proposed for it. As we now know that they argree in this character, and they 

 present no other essential differences aside from mere size, and very slight 

 peculiarities of form, such as are doubtless due to differences of age, we cannot 

 longer regard them as being specifically distinct. 



Perhaps this species is most nearly allied to JV. tuberculatum of Sowerby; at 

 any rate it resembles more nearly Phillips's figure of that species, given in his 

 Geol. Yorkshire, vol. i, pi. 29, fig. 29, than any other form with which we are 

 acquainted. It differs, however, in having its whorls broader, and the most 

 prominent part of their sides, with their nodes, placed nearer the umbilicus; 

 while in a profile view it will be at once distinguished by having its periphery 

 prominently rounded, instead of nearly flat. In addition to this, we have no 

 evidence that N. tuberculatus has a lobe on the inner side of the septa. 



Locality and position: Gravel creek, and other localities in Randolph county. 

 Illinois; Chester division of the Subcarboniferous series. 



