LACUNA. 343 



with frequently a tinge of purplish brown. The sexes are 

 separate. The males are distinguishable from the fe- 

 males by being of a smaller size. This is notoriously 

 also the case with the Crustacea and most of the insect 

 tribe, as well as with many other animals, including our 

 own race. The food of the Littorinidce consists of vege- 

 table matter, either fresh or in various states of putridity. 

 They crawl in a peculiar fashion, moving first one and 

 then the other side of the foot by turns ; the line of such 

 division is marked in the middle of the sole. 



Genus I. LACU'NA^ Turton. PI. VIII. f. 2. 



Body stout : head short : tentacles flattened and smooth : 

 eyes nearly sessile, owing to the smallness of the stalks : foot 

 oval and rounded at each end, with a sharp pointed tail : ojyer- 

 cular apjpendages two, one at each side or corner of the tail. 



Shell more or less channelled or grooved at the base, and 

 slantingly umbilicate : mouth obHquely squarish : pillar rather 

 broad and flattened, so as to receive the channel or groove 

 above mentioned : operculum furnished on the under side with 

 a cartilaginous rib which nearly follows the direction of the 

 spire. 



Da Costa was the first to notice the peculiar charac- 

 ter of the channelled pillar in the shell of Lacuna, 

 finding it difficult to assign his Cochlea parva {Lacuna 

 puteolus of Turton) to any Linnean genus. The only 

 species known to us (four in number) were placed by 

 their respective discoverers in as many diff'erent genera, 

 viz. Turbo, Trochus, Cochlea, and Nerita. They are 

 phytophagous. According to Loven, those which live 

 on brown seaweeds have green bodies, while others 

 found on red seaweeds are rosecolour. They occasion- 

 ally secrete slimy threads (like the Limax arborum), by 

 which they suspend themselves from the frond or stalk 

 * From the excavation of the pillar. 



