Zoological Society. 337 



1. On the Characters of the Genera Pusionella and 

 Clavatula. By J. E. Gray, F.R.S. etc. 



In the List of Genera of Mollusca published in the Proceedings for 

 1848, I gave the name of Pusionella to a genus of shell, referring to 

 the Nefal of Adanson and the Murex pusio of Bom as the type. 



This genus is easily characterized by the smooth thin periostraca, 

 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, with the nucleus or first- 

 formed lamina placed on the straight front or inner side of the oper- 

 culum, wliich 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 was rewarded by the discovery that 

 Pleurotoma bicarinata, which is very nearly allied in form to P. coro- 

 nata, the type of the genus Clavatula 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 

 PleurotomxB, 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 be considered necessary to 

 separate from Pleurotoma the species which have a very short ante- 

 rior canal, which have hitherto been regarded as Clavatulce, they may 

 be called Brillife, as that was the name wliich was first applied to 

 them before they were confounded with the true ClavatulcB. 



These observations show the importance of studjang 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. 



Ann. ^ Mag. N. Hist. Ser. 2. Vol.\'\i. 22 



