272 MANUAL OF THE MOLLUSCA. 
pedicel on each side, but no eyes; foot small, secreting a float 
composed of numerous cartilaginous air-yesicles, to the under 
surface of which the ovarian capsules are attached. Lingual 
ribbon, rachis unarmed ; uncini numerous, simple (like scalaria). 
Branchial plumes 2. Sexes separate. 
Distribution, 10 species. Atlantic, Coral sea. 
The ianthine, or oceanic-snails, are gregarious in the open 
sea, where they are found in myriads, and are said to feed on 
the small blue acelephe (velel/a). 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 haye 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 fioats 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 
fully formed shells, and those near the animal were filled with 
eggs. They have no power of sinking and rising im 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The raft, which is much too large to be withdrawn into the shell, 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. Itis built up from glutinous matter secreted by 
the foot.* 
2 Holopea (symmetrica), Hall. 1847. Outer lip sinuated 
near the base. JZ. Silurian, New York. 
FAMILY XJ.—FISSURELLIDA. 
Shell conical, limpet-shaped; apex recurved; nucleus spiral, 
often disappearing in the course of growth; anterior margin 
notched or apex perforated; 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, sym- 
metrical; anal siphon occupying the anterior notch or perforated 
summit of the shell. Lingual dentition similar to trochus.f 
FISSURELLA, Lam. Key-hole limpet. 
Etymology, diminutive of fissura, a slit. 
Type, F. Listeri, Pl. X1I., Fig. 1. 
* Annales des Sciences Naturelles, 1865. 
7 Fissurella is the best gasteropod for comparison with the bivalves; its large gills, 
- placed one on each side, and its symmetrical 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. 
