Mr. W. Clark on the Littorinidse. 359 



variety from locality, varying in number and in the inflation of 

 the volutions, and in colour from pale brown or yellowish white 

 to dark red-brown or horn-colour. I think that one of the light 

 varieties with the tumid volutions is Montagu's Turbo subumbi- 

 licatus, which species appears to be in great obscurity. Having 

 carefully examined the animals of all the varieties, I can decidedly 

 state that they are identical with the typical Rissoa ulvce. I am 

 much inclined to think that the Rissoa Barleei is one of the 

 varieties. 



Rissoa, Freminville. 



Rissoa parva, Mont, et auctorum.^ 

 JR. costulata. 

 R. rufilabris. 

 R. scalariformis. 

 R. interrupta, fyc. 



Animal spiral ; mantle plain, just even with the shell ; the head 

 is a short dark brown muzzle, cloven with a vertical orifice be- 

 neath, the disk of which is yellow; tentacula long, slender and 

 cylindrical, yellow, with a longitudinal row of white detached 

 flakes, and often the reverse, white, with yellow interrupted 

 flakes ; eyes on short offsets at the external bases. Foot flaky- 

 white above and beneath, long, narrow, truncate in front, slightly 

 auricled ; the point is blunt, from whence a small upper lobe or 

 membrane springs, on which is fixed a suboval corneous oper- 

 culum with spiral loose striae, and at its extremity a single short 

 white tentacular filament is seen. The branchial plume consists 

 of 15-18 minute vessels attached under and to the mantle and 

 back of the neck. 



This is another of the genera the conchologists have taken 

 possession of for the manufacture of varieties into species. The 

 type Rissoa parva is a most variable species, resulting from loca- 

 lity and other causes. The animals of all the varieties enume- 

 rated above are identical ; we have examined them all, and can 

 find no appreciable difference except varieties of colour and oc- 

 casional variations in the length of the terminal filament, which 

 is never in any two selected shells precisely similar. The R. 

 costulata is an elongated variety; the R. rufilabris a short 

 tumid one, with a red or pink peristome, which may often be 

 seen in its congeners. The R. interrupta is a more slender, thin, 

 less plicated, dwarf variety. The R. scalariformis has the plicae 

 more numerous, white and delicate ; and lastly, the typical Rissoa 

 parva varies so much that it is difficult to find two shells alike. 

 These varieties live in company in the lower levels of the littoral 

 zone, but are more plentiful at the borders of the laminarian 

 district. 



