Mr. W. Clark on the Littorinidae. 357 



without any particular concurrence therein^ recorded these opi- 

 nions in their interesting work. But in corroboration of these 

 views I shall adduce some considerations on another genus, 

 which has largely contributed to the increase of pseudo-species ; 

 T allude to that portion of the genus Littorina designated L. lit- 

 torea and L. rudis. 



I now take a rapid glance at the family of the Littorinida, and 

 then conclude my paper by giving the notes and specific descrip- 

 tions of the types of each genus as proofs, and in support of what 

 I have advanced, and to afford data to malacologists to come to 

 just conclusions on what I have submitted to their consideration. 



LlTTORINID^. 



This well-known family as now constituted consists of the 

 genera Lacuna, Assiminia, Rissoa, and Littorina. The Lacun(e of 

 authors are scarcely distinguishable from the Littorina littoralis] 

 the animals are all but identical, and the two genera appear to be 

 separated on conchological grounds, — the groove in the columella 

 rather than an animal distinction. I think it would conduce to 

 the interests of science if the Lacunce were merged in the genus 

 Littorina, and marched pari passu with Littorina littoralis as 

 species. 



I shall show malacologically, in a memoir I am preparing on 

 the Lacunce, that the difference of the animal of Lacuna and Lit- 

 torina littoralis is scarcely appreciable. The Lacunce have only 

 one or two species instead of the eight or ten of authors. 



Assiminia was established by Mr. Gray, and differs chiefly from 

 the Rissoa ulvcs and R. subumbilicata of Montagu, in having the 

 e3^es fixed at the extremities of the tentacula, in consequence of 

 being placed on adnate pedicles of concurrent length. 



The Rissoce are very minute ; they inhabit the littoral and la- 

 minarian zones ; the types of the first zone are the Rissoa ulvce 

 and R. subumbilicata, and of the second R. parva. These are 

 true littoral Rissoce. Many minute shells which conchologically 

 appear Rissoce inhabit only the coralline zones, and as most of 

 the animals are unknown, the true relations between the ani- 

 mals of the littoral and coralline districts are not as yet satisfac- 

 torily ascertained. 



Littorina has only four species, L. littorea, L. rudis, L. neri- 

 t aides, and L. littoralis. 



I now proceed to the specific descriptions : — 



Assiminia, Gray and Leach. 

 Assiminia Gray ana. 

 Animal spiral, yellowish brown ; mantle simple ; head with a 



