76 THE NAUTILUS. 



anterior cicatrices smooth, well-impressed and distant ; posterior 

 cicatrices sliglitly impressed, smooth and confluent ; nacre pale blue, 

 white or coppery. Width, 2| ; length, 1| ; diameter, T 9 ^ inches. 



Habitat : Spring Creek, Decatur Co., Ga. 



Type lot in National Museum. 



Remarks : Affinity, Unio scatn<itns Morel. The roughness of the 

 surface of this singular shell is the one feature that at once arrests 

 attention. We know of no other U. S. species of this class (as to 

 outline) that possesses such a surface. It, is not, however, so con- 

 spicuously furrowed as its affinity. 



AN ATTEMPT TO DEFINE THE NATURAL GEOUPS OF STROMBS. 



BY GKO. HALCOTT CHADWICK. 



Having recently had the privilege of studying a fine series of the 

 living species of Strombus, Pterocera and Terebellum, the opportunity 

 seemed favorable for a critical examination of the entire group. As 

 I am not aware that any similar attempt has been made to revise the 

 Stromhi. the results obtained are respectfully submitted to the scien- 

 tific world. Lacking anatomical data, I have freely used neglected 

 and sometimes indefinable features of the shells as guides to their 

 inter-relationships. Forced to recognize some new groups, I have 

 left the secondary matter of names to those more skilled in nomen- 

 clature, but have indicated the probable application of the titles 

 already in use, as to the types of which scarcely any two authors, save 

 Tryon and Fischer, agree. 



Of the species and varieties of Xtrombtis recognized in Tryon's 

 monograph, I have seen all but /\ fntct/fr, taunts, listen', dilatatus, 

 labiosxs, dcfortnis, septhmis, ntiiit'iims, siblxddi, erytln iirns (true), }>nl- 

 chellt/s, /iet//i, scalariformis and bttlbidns, although S. goliath, fusifor- 

 mis and terebellutits were not now before me. All ten of the Ptero- 

 certe were at hand. 



The grouping of the species which seems to me most in accordance 

 with their natural relations, as indicated by shell features and geo- 

 graphical distribution, is as follows: 



GROUP A (Monodactyhis}. 



Canal lengthened, bent abruptly dorsally; lip with margin straight- 

 ened, incurled, truncated at both ends, bearing a posterior digitation 



