152 PLEUROTOMIt)^:. 



been constantly separated into different groups ; and of a large 

 proportion of the species, the proper relationships have not and 

 cannot be worked out from accessible material. In no other 

 family of shells have these groups been so hastily proposed or 

 with so little data upon which to found them ; yet so generally 

 have they been adopted that to destroy ill-founded groups and 

 unite the species under the one generic name Pleurotoma, would 

 cause such a large duplication of specific names, and consequently 

 so much alteration of these latter, that it appears better, in the 

 interest of science, to retain some of these genera. 



In the " Structural and Systematic Conchology " I have 

 recorded thirty-three groups, sections or subgenera under the 

 genus Pleurotoma, and without indicating any difference of rank 

 or grade among these ; here I shall be compelled to arrange them 

 into subfamilies, genera, subgenera and sections, supplied with 

 characters only exhibited by selected typical species, and even 

 in these of much inferior importance to those upon which similar 

 divisions are founded in most other families of mollusks. The 

 three subfamily characters, the presence or absence of an oper- 

 culum, and the position of the nucleus when the operculum is 

 present, would be far from having similar rank in many other 

 families of marine mollusks ; it only needs to refer to the Muricidse, 

 Tritonidae, etc., where the position of the nucleus is considered 

 of only subgeneric importance ; to the Buccinidae, in one genus 

 of which the operculum is indifferent!}^ absent or present in the 

 same species. This character becomes of still less value to us 

 because the operculum has so seldom been preserved that in 

 more than half of the species it has neither been figured nor 

 described ; and because there are no other characters from which 

 that of the operculum can be predicated. 



In " Structural and Systematic Conchology," vol. ii, p. 50, 

 will be found Dr. Gray's account of the reproduction of a lost 

 portion of an operculum in Pleurotoma babylonica; the operculum 

 which normally has an apical nucleus, by this restoration having 

 concentric lamellae from the middle. 



The other systematic characters the length of the canal, 

 position of the sinus, surface of the embryonic whorls, sculpture, 

 etc. are equally unreliable. It is not surprising that in groups 

 so vaguely defined the personal equation should be more than 



