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FISSURELLID®. 
Famity FISSURELLID%. 
Shell conical, limpet-shaped ; apex recurved; nucleus spiral, 
often disappearing in the course of growth ; anterior margin 
notched or apex perforated; muscular impression horseshoe- 
shaped, open in front. 
Tongue with a central median tooth, five denticulated uncini, 
and numerous slender, hooked laterals. Body broad and conical. 
Head with a short, wide muzzle; tentacles subulate, with the 
eyes on slightly elevated tubercles at their external. bases. 
Mantle- -margin fissured in front, the free. edges forming an anal 
siphon occupying the anterior fissure, or perforation in the apex 
of the shell; gills two, symmetrical, on the back of the. neck. 
Foot dilated, sides with the upper part furnished with a series 
of short cirri or rudimentary filaments. Operculum none. 
Phytophagous and littoral. The nucleus of the shell is always 
spiral, although in the adult the shell. assumes a conic form; in 
some genera there is a rudimentary spire in the young state, 
which disappears in the course of growth. 
FIssuRELLA, Lam. 
Etym.—Diminutive of fissura,.a slit. 
Distr.—125 sp. Universal, but mostly in warm seas. Fossil, 
30 sp. Carboniferous—. F. picta, Gmel. (1xxxiii, 15). 
Shell oval, conical, depressed, with the apex in front of the 
centre, and perforated ; surface radiated or cancellated ; muscu- 
lar impression with the points incurved. 
In very young shells the apex.is entire and subspiral; but as 
the perforation increases in size, it encroaches on the summit, 
and gradually removes it. The key-hole limpets are locomotive; 
returning, however, to their accustomed station, the rocky sur- 
face of which is worn into an impression corresponding with the 
outline of the shell. 
CREMIDES, H. and A. Adams. Surface rugose, muricated or 
cancellated ; the lip denticulated. &. nodosa, Born (1xxxiii, 16). 
FISSURIDEA, Swainson. Shell subconic, capuliform, the poste- 
rior summit narrowly perforated.  F. pileopsoides, Reeve 
(Ixxxiv, 30). 
LUCAPINA, Gray. Shell oval-oblong, conic, depressed, caneel- 
lated; summit subcentral, the oval opening surrounded by a 
callus; margin crenulated. /. crenulata, Sowb. (1xxxili, 17). 
Mantle-margin. fimbriated, reflexed. more or. less over the 
edges of the shell. This group includes all those reticulated 
and cancellated forms generally referred to Fissurella, which 
have the margin of the aperture crenulated, and in which the 
perforation, internally, is transversely truncate and sometimes 
slightly concamerated. 
