RISSOA. 355 



are very dark ; it carries a light, corneous, suboval, pauci- 

 spiral operculum, with fine oblique striae. The foot is per- 

 fectly rounded behind, and in almost all specimens more or 

 less emarginate, though in some scarcely visible, in others 

 decidedly so, at several points of the arcuation ; it is truncate 

 and well auricled in front, without a medial line. There is 

 invariably a very short cylindrical process or fillet exserted 

 from that part of the mantle which lines the anal canal or 

 upper angle of the aperture : I am unable to say anything as 

 to its use : whether such an appendage exists in other minute 

 congeneric species, and from its minuteness has passed with- 

 out observation, must be left for future examination. I con- 

 sider this species a Rissoa. Though it has not the caudal 

 filament and pointed pedal termination of the typical Rissoce, 

 we must not forget that there are some Rissoee with rounded 

 tails and without a caudal cirrhus at the termination of the 

 operculigerous lobe. 



I have elsewhere said, that every British Rissoa must have 

 a distinct genus, if we insist on a literal agreement with the 

 type R. parva ; they all differ as much, or even more, from 

 it, than the R. ulvee ; indeed, this species only varies in the 

 broader foot, its rounded posterior termination, and in being 

 without a cirrhus. Some authors deposit the R. nlvte, with 

 one or two more species of the estuaries, in Pfeiffer's Palu- 

 dinella : surely this is sad generic refinement. Since the 

 observation above, on the filament springing from that part 

 of the mantle which lines the upper angle of the aperture, I 

 can report, that it is present in almost all the Rissoae, but its 

 precise use is not yet satisfactorily made out. 



R. PARVA, Da Costa. 



R. parva, Brit. Moll. iii. p. 98, pi. 76. f. 2, 6 ; pi. 77- f. 6, 7 ; pi. 82. 



f. 1-4. 



R. interrupta, Brit. Moll. iii. p. 100 ; et Auct. 

 R. cosfulata, Alder. 



, Brit. Moll. iii. p. 103, pi. 77- f. 4, 5. 



R. rufilabris, Alder. 



-, Brit. Moll. iii. p. lOfi, pi. 77- f. 8, 9. 



2 A 9 



