GASTEROPODA. 149 



each side, but no eyes ; foot small, secreting a float composed of numerous 

 cartilaginous air-vesicles, to the under surface of wMch the ovarian capsules 

 are attached. Lingual ribbon, rachis unarmed ; uncini numerous, simple 

 (like scalaria). Branchial plumes 2. Sexes separate. 



Bistr., 6 sp. Atlantic, Coral sea. 



The ianthinre, or oceanic-snails, are gregarious in the open sea, where 

 they are found in myriads, and are said to feed on the small blue acalephfe 

 {velella). They are frequently drifted to the southern and western British 

 shores, especially when the wind continues long from the S.W. ; in Swansea 

 bay the animals have been found quite fresh. "When handled they exude a 

 violet fluid from beneath the margin of the mantle. In rough weather they 

 are driven about and their floats broken, or detached, in which state they ai-e 

 often met with. The capsules beneath the further end of the raft have been 

 observed to be empty, at a time when those in the middle contained young 

 with fully formed shells, and those near the animal were filled with eggs. 

 They have no power of sinking and rising in the water. The raft, which is 

 much too large to be withdra\\Ti into the shell, is an extreme modification of 

 the operculum. 



FAMILY XI. FissuRELLiD^. 



Shell conical, limpet- shaped ; apex recm-ved ; nucleus spiral, often dis- 

 appearing in the course of growth ; anterior margin notched, or apex perfo- 

 rated ; muscular impression horse-shoe shaped, open in front. 



Animal with a well-developed head, a short muzzle, subulate tentacles, 

 and eyes on rudimentary pedicels at their outer bases ; sides ornamented with 

 short cirri ; branchial plumes 2, symmetrical ; anal siphon occupying the an- 

 terior notch or perforated summit of the shell. Lingual dentition similar to 

 trochus.* 



FissuRELLA, Lam. Key-hole limpet. 



Btym,, diminutive oifissura, a slit. 



Ti/pe, F. Listen. PI. XI., fig. 1. 



Shell oval, conical, depressed with the apex in front of the centre and 

 perforated ; sm-face radiated or cancellated ; muscular impression with the 

 points incm-ved. 



In very young shells the apex is entire and sub-spiral ; but as the perfo- 

 ration increases in size it encroaches on the smnniit and gradually removes 

 it. The key -hole limpets are locomotive ; they chiefly inhabit the laminarian 

 zone, but range downwards to 50 fins. 



Distr., 120 sp. America, Brit., S. Africa, India, China, Australia. U. 

 California — Cape Horn. 



* Fissurella is the best gasteropod for comparison with the bivalves; its large giUs, 

 placed one on each side, and its symmetrical shell, pierced with a median orifice for 

 the escape of the out-going branchial current, are unmistakeable indications of homo- 

 logies with the lamelli-branchiata. See p. 48. 



