This genus is easily characterized by the smooth thin periostraea, 
and the sharp-edged oblique plait which crosses the lower part of the 
canal. At the time I formed the genus, which contains several spe¬ 
cies in my collection, all coming from Africa, I was convinced that it 
was separate from the other zoophagous mollusca, from the characters 
assigned to it above, though I am aware that several zoologists were 
inclined to consider that they were scarcely sufficient for the forma¬ 
tion of a generic group. 
The examination of the operculum of the shells arranged in this 
group has shown that it affords a most excellent character, which 
separates it at once from all the other genera of the family. The 
operculum is formed of concentric laminae, w T ith the nucleus or first- 
formed lamina placed on the straight front or inner side of the oper¬ 
culum, which is situated next to the pillar of the shell. With this 
peculiarity the genus must now be regarded as firmly established. 
This form of operculum had only before been observed in the genus 
Bezoardica. 
The discovery of this character in shells which had been regarded 
by most authors as Fusi, induced me to examine the opercula of 
some other allied genera, and I w T as rewarded by the discovery that 
Pleurotoma bicarinata, which is very nearly allied in form to P. coro- 
nata , the type of the genus Clavatida of Lamarck’s ‘ System,’ has 
the operculum of the same shape and formed nearly in the same man¬ 
ner as that of the genus Pusionella ; while Pleurotoma Babylonica , 
P. Virgo, and P. oxytrophis, which may be regarded as the typical 
Pleurotomce, have the ovate lanceolate operculum with the nucleus on 
the acute apex, like the typical Fusi. 
This being the case, it appears to me desirable that the genus Cla- 
vatula should be re-established, and restored to the species which has 
the operculum of this kind. Should it he considered necessary to 
separate from Pleurotoma the species which have a very short ante¬ 
rior canal, which have hitherto been regarded as Clavatiilce, they may 
be called Drillire, as that was the name which was first applied to 
them before they were confounded with the true Clavatulce. 
These observations show r the importance of studying the opercula 
of the different genera; and I may add, that the attention which I 
have been able to bestow on the subject has convinced me that they 
form quite as important a character for the distinction of the genera, 
and the arrangement of the genera into natural groups, as the struc¬ 
ture and form of the shelly valve, or of the external form of the ani¬ 
mals themselves; and this may well be believed, when we consider 
them, as I am inclined to do, as an imperfectly developed valve, and 
as homologous to the second valve of the bivalve shell. 
