326 fissurellid^. 



Family 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, sj^mmetrical, 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 Jissura, a slit. 



Distr. — 125 sp. Universal, but mostly in warm seas. Fossil, 

 30 sp. Carboniferous — . F. picta, Gmel. (Ixxxiii, 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. F. nodosa^ Born (Ixxxiii, 16). 



FissuRiDEA, Swainson. Shell subconic, capuliform, the poste- 

 rior summit narrowly perforated. F. pileopsoides, Reeve 

 (Ixxxiv, 30). 



LUCAPiNA, Gray. Shell oval-oblong, conic, depressed, cancel- 

 lated ; summit subcentral, the oval opening surrounded by a 

 callus; margin crenulated. F. crenulata, So'w'b. (Ixxxiii, 1*7). 



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. 



