152 PLEUROTOMIDA: 
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 Muricide, 
Tritonide, etc., where the position of the nucleus is considered 
of only subgeneric importance ; to the Buccinide, in one genus 
of which the operculum is indifferently 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 lamellz 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 
