168 HELICIDiE. 



lus uniting its upper and lower terminations ; but these 

 are common in S. concava. Of numbers which I have 

 examined, the greater part have the depression, and all 

 except the young have the uniting callus. 



The differences observable are the following : the 

 size of this shell greatly exceeds the former in all its pro- 

 portions, its transverse diameter being nearly twice as 

 great. This difference is not caused by an increased 

 number of whorls, for the number in both is precisely 

 the same ; but this shell seems to be projected originally 

 upon a larger scale, the nucleus being as much larger as 

 mature specimens. The color is a light greenish, or 

 rusty yellow. The umbilicus is not so widely expanded, 

 and does not admit of counting all the whorls ; and the 

 whorls seem to be more voluminous. The striise of 

 growth are usually coarser. 



When specimens of this shell shall be obtained in suffi- 

 cient numbers for comparison, the question of its identity 

 with the other can be satisfactorily settled. In the 

 mean time, it will probably be received as a good spe- 

 cies. The color of the animal, also, as given by Mr. 

 Drayton in the drawings of the Exploring Expedition, 

 from which our description is derived, as well as the 

 form of the foot, go to estabhsh the difference in ques- 

 tion. 



