Ml'. W. Clark on the Chemnitzia opalina and C. diaphana. 295 



on the neck, at the bases of the tentacula in a line with them, 

 sufficiently indicates its parentage, if all othex* characters were 

 absent. I challenge the production of an animal of the Lit- 

 torinidce with a spiral shell and operculum that has any ana- 

 logy to this in respect of the eyes; but the position of these 

 organs is one of the great distinctive characters of the Pyra- 

 midellidce. I may just observe, that R. opalina has alliance 

 with Truncatella by the muzzle and short tentacula. All my 

 shells have the true button-shaped sunken subreflexed apex of 

 that section of the Chemnitzm which is represented by the 

 dwarf littoi'al Chem. rissoides, only it is somewhat more bent on 

 the second volution than in that species : the pillar-lip, if care- 

 fully examined, will show sometimes decided folds, but usually, as 

 is often the case in Chem. obliqua, the " decorata " of Mr. Bean, 

 they do not force themselves within the limits of visibility. I 

 have dissected the columella of several examples of this species 

 as well as of C. diaphana, and I find them with the lax elongated 

 spiral wreath exactly as in C. obliqua, with which they are strictly 

 congeneric in almost all points. The operculum, though modi- 

 fied in the shape of the striae, is decidedly of Chemnitzian type ; 

 that of the Littorinidce is usually spiral. The double tentacula 

 of the Chem. opalina are only the broad longitudinally folded 

 ones of some of the Chemnitzia qualified by scission. AH these 

 circumstances satisfy me that this animal is a true Chemnitzia, 

 and as it is congeneric with Chem. diaphana, mihi, Jeffreysia dia- 

 phana, Alder, that disputed species will fall into the same cate- 

 gory as this, I therefore again express an opinion that the so- 

 called genus Jeffreysia is superfluous, inasmuch as an appropriate 

 one is already formed for its species. 



I am now preparing for a lengthened absence from Bath at 

 Exmouth ; in the autumn I hope to have it in my power to con- 

 vince either Mr. Alder or myself of some important malacological 

 facts. 



I am. Gentlemen, your most obedient servant, 



William Clark. 



Bath, March 16, 1851. 

 Postscript. — I have ascertained that the opercula of probably 

 all the Chemnitzia are undoubtedly characterized by an apophysis 

 or process, with slight specialty modifications of shape and posi- 

 tion, which has been considered a unique incident of the so- 

 called genus Jeffreysia diaphana of Mr. Alder, — my Chemnitzia, — 

 which in many species has nearly similar furrows and markings, 

 both on the under and upper surface, and inflexion on the upper 

 range next the pillar ; and it would be strange if it were not so, 

 as I will prove that the Jeffreysia diaphana is a true Chemnitzia. 



