272 MANUAL OF THE MOLLUSCA. 



pedicel on each side, but no eyes ; foot small, secreting a float 

 composed of numerous cartilaginous air-vesicles, to the under 

 surface of which the ovarian capsules are attached. Lingual 

 ribhon, rachis unarmed ; uncini numerous, simple (Hke scalaria). 

 Branchial plumes 2. Sexes separate. 



Distribution, 10 species. Atlantic, Coral sea. 



The ianthinae, or oceanic-snails, are gregarious in the open 

 sea, where they are found in myriads, and are said to feed on 

 the small blue acelephse [velella). They are frequently drifted 

 to the southern and western British shores, especially when the 

 wind continues long from the south-west ; 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 are often met with. The capsules 

 beneath the farther end of the raft have been observed to be 

 empty, at a time when those in the middle contained young with 

 fally 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 The raft, which is much too large to be withdrawn into the shell, 

 is generally thought to be an extreme modification of the oper- 

 culum ; but M.. Lucaze-Duthiers, who has seen the raft formed, 

 denies this. It is built up from glutinous matter secreted by 

 the foot.* 



? Holopea (symmetrica), Hall. 184Y. Outer lip sinuated 

 near the base. L. Silurian, New York. 



PAMILT XI. — FlSSTJRELLID^. 



Shell conical, limpet-shaped ; apex recurved ; nucleus spiral, 

 often disappearing in the course of growth ; anterior margin 

 notched or apex perforated ; muscular imj)ression 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, sym- 

 metrical ; anal siphon occupying the anterior notch or perforated 

 summit of the shell. Lingual dentition similar to trochus.'f 



FissTJEELLA, Lam. Key-hole limpet. 



Etvmology, diminutive of fissura, a slit. 

 Type, P. Listeri, PI. XL, Fig. 1. 



* Annales des Sciences Naturelles, 1865. 



t Fissurella is the best gasteropod for comparison with the bivalves ; its large gills, 

 placed one on each side, and its sj'mmetrical shell, pierced with a median orifice for the 

 escape of the out-going branchial current, are unmistakable indications of homologies 

 with the lamelli-branchiata. See p. 39. 



