MOLLUSCA: GASTEROPODA. 295 



Fam. 2. Pyramidellidcz. Shell turreted, with a small aperture, 

 sometimes with one or more prominent plaits on the columella. 

 Operculum horny, imbricated. 111. Gen. Pyramidella^ Ckemnitzia, 

 Eulima. 



Fam. 3. Cerithiada. Shell spiral, turreted ; aperture channelled in 

 front, with a less distinct posterior canal. Lip generally expanded 

 in the adult. Operculum horny and spiral. 111. Gen. Cerithium, 

 Potamides, Aporrhais. 



Fam. 4. Melaniadce. Shell spiral, turreted ; aperture often chan- 

 nelled or notched in front ; outer lip acute. Operculum horny 

 and spiral. 111. Gen. Melania, Paludomus. 



Fam. 5. Turritellidce. Shell tubular, or spiral, often turreted ; 

 upper part partitioned off; aperture simple. Operculum horny, 

 many-whorled. Foot very short. Branchial plume single. 111. 

 Gen. Turritella, Vermetus, Scalaria. 



Fam. 6. Littorinidce. Shell spiral, top-shaped, or depressed ; aper- 

 ture rounded and entire ; operculum horny and pauci-spiral. 111. 

 Gen. Littorina, Solarium, Rissoa, Phorus. 



Fam. 7. Pahtdinidiz. Shell conical or globular ; aperture rounded 

 and entire ; operculum horny or shelly. 111. Gen. Paludina, 

 Ampullaria, Valvata. 



Fam. 8. Neritida. Shell thick, globular, with a very small spire ; 

 aperture semi - lunate, its columellar side expanded ; outer lip 

 acute. Operculum shelly, sub-spiral. 111. Gen. Nerita, Pileolus, 

 Neritina. 



Fam. 9. Turbinida. Shell turbinated (top-shaped), or pyramidal, 

 nacreous inside. Operculum horny and multi-spiral, or calca- 

 reous and pauci-spiral. 111. Gen. Turbo, Trochus, Delphinula t 

 Euomphalus. 



Fam. 10. Haliotida. Shell spiral, ear-shaped, or trochoid ; aperture 

 large, nacreous. Outer lip notched or perforated. No operculum. 

 Mantle-margin with a posterior fold or siphon, occupying the slit 

 or perforation in the shell. Metapodium rudimentary. 111. Gen. 

 Haliotisy Scissurella, Pleurotomaria, Murchisonia, lanthina. 



Fam. 1 1. Fissurellida. Shell conical, patelliform, with a notch in 

 the anterior margin, or a perforation at its apex, which is occupied 

 by an anal siphon. Muscular impression horse-shoe-shaped, open 

 in front. 111. Gen. Fissurella, Emarginula, Parmophorus. 



Fam. 12. Cafyptrceida. Shell patelliform, with a more or less spiral 

 apex ; interior simple, or divided by a shelly process to which the 

 adductor muscles are attached. 111. Gen. Calyptnza, Pileopsis. 



Fam. 13. Patellidtz. Shell conical, with the apex turned forwards; 

 muscular impression horse-shoe-shaped, open in front. Foot as 

 large as the margin of the mantle. Respiratory organ in the form 

 of one or two branchial plumes, lodged in a cervical cavity ; or of 

 a series of lamellae surrounding the animal between the body and 

 the mantle. 111. Gen. Patella, Acmcea. 



Fam. 14. Dentalida. Shell tubular, symmetrical, curved, open at 

 both ends. Aperture circular. Foot pointed, with symmetrical 

 side-lobes. Gen. Dentalium* 



Fam. 15. Ckitonida. Shell multivalve, composed of eight transverse 



* Dentalium is placed by Professor Huxley amongst the Pteropoda, from 

 its rudimentary head, the neural flexure of the intestine, the nature of the 

 epipodia, and the characters of the larva. 



