394 MOLLUSC A. 



small cornea. The radula lias multiple centrals. The ctenidia are 

 bipectinate and free anteriorly. Traces of bilateral symmetry are 

 usually present. There are generally two auricles and two kidneys, 

 which open on short papillae. The gonad (Fig. 297 b) opens into 

 the right kidney (except in the Neritidae). 



Tribe 1. DOCOGLOSSA. 



Nervous system without dialyneury ; eyes open without lens. Two osphradia. 

 Operculum and hypobranchial glands absent. Jaw unpaired and dorsal. Radula 

 has strong, pillar-shaped teeth, and never more than three marginals on each 

 side. A single auricle ; the rectum does not traverse the ventricle or the 

 pericardium. The visceral sac is conical in the adult, and not twisted. 



Fam. 1. Acmaeidae. Left ctenidium only ; it is to a great extent free. 

 Acmaea Esch. ; Scurria Gray; Addisonia Dall. 



Fam. 2. Patellidae. Limpets. Ctenidia absent, replaced by a circlet of 

 plate-like structures in the mantle-groove. Patella L. ; Heltion Gray. 



Fam. 3. Lepetidae. Ctenidia, pallial branchiae, and eyes absent. Lepeta 

 Gray. 



Tribe 2. RHIPIDOGLOSSA. 



Nervous system with dialyneury; eyes with lens; a single osphradium (the 

 left), except when there are two ctenidia; one or two hypobranchial glands. 

 Jaws paired ; marginal teeth of radula numerous, and crowded together like 

 the ribs of a fan (Fig. 293 A}. A crop, oesophageal gland, and a stomach 

 caecum (often spiral) present. Two auricles, ventricle traversed by rectum 

 (except in Helicinidac). Often with an epipodial projection, which is frequently 

 tentaculiferous, on each side of foot. 



Section A. With two ctenidia (Zygobranchiatc). Shell with marginal slit or 

 holes corresponding to an anal opening in mantle. 



Fam. 1. Pleurotomaridae. Visceral mass and shell twisted. Mantle slit in 

 front in the middle line. Operculate. Cambrian onwards. Pleurotomaria 



Defrance (Fig. 304 A); P. quo- 

 yana Gulf of Mexico ; Scissur- 

 ella d'Orb. ; Trochotoma Desh. 



Fam. 2. Haliotidae. Spire 

 of the visceral sac and shell 

 much reduced. Shell flat, with 

 wide aperture, with internal 

 mother-of-pearl lustre and a 

 row of holes on the left side 

 FIG. 311. Fissurella maxima (from Bronn). over the mantle slit. The 



mantle - cavity is displaced to 



the left side by the enormous spindle -muscle. Epipodium tentaculiferous. 

 Without operculum. Haliotis L., ear shell, ormer (Figs. 296 and 304 D), 

 adherent to rocks like a limpet ; found in large numbers in the Channel Islands. 

 Fam. 3. Fissurellidae. Visceral sac and shell conical, with an apical 

 aperture or an anterior marginal excavation. Limpet-like. Without oper- 

 culum. Fissurella Brug., key-hole limpet (Figs. 304 F, and 311) ; Pimcturella 

 Lowe ; Pupillia Gray ; Emarginula Lam., with anterior marginal excavation ; 



