PODOPHTHALMA. 377 



Some of the families have the eyes on peduncles separate 

 from the tentacles, and the sides of the foot furnished 

 with a continuous fringe bearing tentacular filaments, as 

 the TrocJiidce ; the Neritidce have pedunculated eyes, 

 but want the lateral fringe ; while in the Fissurellidce the 

 eyes are sessile, and the lateral fringe is rudimentary. 

 The gills are usually in one or two series over the back 

 of the neck, but in the PatelUda and ChitonidcB they 

 are under the edge of the mantle round the foot. The 

 shell is spiral in the Nerltidce and Trodiidce ; patelliform 

 in the Fissurellidie, Gadiniidm, Tecturidce, and Patel- 

 lidcp; and composed of several pieces in the ChitonidcE. 



Sub-order PODOPHTHALMA. 



Eyes pedicelled, pedicels separate from the tentacles ; 

 teeth numerous, the lateral uniform, very numerous, 

 crowded. Shell and operculum spiral. 



Fam. NERITID^. 



Lingual dentition very similar to that of the Trochidce, 

 the central teeth few, the lateral hooks, or uncini, very 

 numerous. Head with a broad, short muzzle ; tentacles 

 slender and subulate, with the eyes on stout peduncles 

 at their outer bases ; no head-lobes or neck-lappets. 

 Foot oblong, triangular, the sides simple, without fila- 

 ments or lateral membrane. 



Operculum articulated, shelly, sub-spiral. 



Shell depressed or oval, not umbilicated ; spire very 

 short, cavity simple, from the absorption of the inter- 



VOL. I. -3 c 



