102 THE NAUTILUS. 



A NOTE UPON THE INSUFFICIENCY OF THE OPERCULUM AS A BASIS 

 OF CLASSIFICATION IN ROUND-MOUTHED SHELLS. 



(Continued). 



BY L. P. GRATACAP. 



The naturalists who subsequently worked upon the classification 

 of the mollusca were all sensibly influenced by these observations, 

 and the operculum became a diagnostic note in the separation of 

 genera. Troschel (1847) and Pfeiffer (1852), thoroughly applied 

 this method of discrimination, and augmented the number of genera. 

 M. Petit de la Saussaye (1850) questioned the significance of the 

 operculum as a real generic feature, and especially so far as the 

 nature of the substance of the operculum had been used by Pfeiffer 

 to distinguish genera. ' He says (Journal de Conchyliologie, Vol. I., 

 1850) "the nature more or less calcareous or corneous of the oper- 

 cula arises rather from accidental circumstances, such as the habita- 

 tion of these animals, their nourishment, the character of the ground 

 upon which they live, the force of the sun, etc. As to the variable 

 form of the volutions of the spire, observed in these accessory parts, 

 it doubtless arises from the modifications, that the tissue secreting 

 them presents ; modifications which do not seem adequate for the 

 establishment of generic groups." 



Benson reiterated the importance of the operculate features of the 

 round-mouthed shells, and the system of classification incorporated 

 in H. and A. Adams, " Genera of Recent Mollusca" (1858) still 

 further imbedded in the science the critical relations of the form 

 and substance of the operculum to the natural limitations of the 

 genera. 



In the latest important systematic treatise, Tryon's Structural and 

 Systematic Conchology, the operculum is perhaps less exhaustively 

 used for descriptive purposes, but the author remarks that the oper- 

 culum " presents many beautiful modifications of structure, character- 

 istic of the smaller groups, which are often peculiar to limited 

 regions as in the Helicidae." 



In an examination of this group of shells in the general collection 

 of the American Museum of Natural History and representing the 

 Jay, Haines, and Constable cabinets, the somewhat exaggerated im- 

 portance of the separative features of their opercula seemed apparent. 



