296 Mr. W, Clark on the Chemnitzia opalina and C. diaphana. 



With respect to the variation of the shape of the strise of the 

 operculum in Chem. diaphana, I may observe, it is not uncommon 

 in the same genus in other families, and may be seen even in the 

 same species ; I have Trochi with the apparently fine annular striae, 

 and others with radiating lines, and as grossly spiral as in the 

 Littorina Uttorea. I have already stated that the operculum in 

 Chemnitzia shows much variation ; I adduce as an example, that 

 in some specimens of the C. rufa, part of the area is coarsely sub- 

 annular, with stripe on the other part, radiating from the elliptic 

 curves. The fact of a process in the Chemnitzia is placed beyond 

 doubt, as I have examined foui-teen species, having with great 

 trouble cleansed the opercula of my dried specimens from every 

 particle of animal matter ; a difficult task from their minuteness, 

 but of the highest importance for correctly viewing the process 

 and other characters. I have placed them all on tablets, and shall 

 be glad to show them to any competent observer; they are 

 interesting from the fact that Chemnitzia is the only marine 

 gasteropodan genus which has these peculiar characters. The 

 apophysis is nothing more than an extension, sometimes from 

 the margin, but more usually from the under surface of the 

 operculum, of one of the callosities with which it is generally 

 studded, assuming the modifications of shape dependent on them ; 

 it is usually situate opposite the nucleus, and is often connected 

 with it ; the process in some species appears to be of a subtes- 

 taceous character. 



We will now see what Mr. Alder says of this supposed peculiar 

 process in his Jeffrerjsia diaphana ; I quote his words : " The oper- 

 culum is very peculiar; the projecting internal plate I do not re- 

 collect to have obsened in any other genxis, though the spine in 

 Nerita approaches to it." (Alder in Brit. MoUusca, vol. iii. p. 152.) 

 It is proper to observe, that no operculum of the Littorinida has 

 the least similarity to the present one, conseqviently Jeffreysia, as 

 Mr. Alder thinks, cannot belong to that family. 



Mr. Alder's figures of the opercula are very incon-ect, particu- 

 larly that of the C. rissoides, in which \h.Q process and rough under 

 sculpture are omitted. It is strange Mr. Alder has forgotten to 

 mark the apophysis, which in Chem. rissoides is quite as apparent 

 in proportion to its minor size as in C. diaphana — though not 

 large, it is sufficiently visible : I have two dissected specimens. 



The apophysis is strikingly conspicuous in Chem. conoidea and 

 Chem.plicata, much more so than in Chem. diaphana or its congener 

 Chem. opalina ; in Chem. acuta it approaches nearly to the two 

 latter species, but is not quite so marginal ; in C, spiralis, C. de- 

 cussata and C. interstincta , its position and the rib are all but 

 identical with C. diaphana and C. opalina. This fact of the es- 

 sential identity of the operculum of Chemnitzia and the Jeffreysia, 



