Introduction to Aniinal JMorplwlogy. 307 



facets, a posterior flat pyramid with an umbo, and a pair of 

 lateral areas ; each of these consists of a tegmeiitiim under the 

 epidermis ; free between the mantle edge, and an articulamen- 

 iiim, deeper layer ; there are two pair of joints, ariiculi antici z.nd 

 posiici, along the edge of the lateral areas. There is but one 

 family. 



§ II. Cyclobranchiata( Docoglossa) — Limpets, with coni- 

 cal, inoperculate shells ; tentacles two, with eyes at their bases ; 

 foot large and flat ; radula with no middle plate, and ridged 

 lateral and intermediate plates. This includes the family 

 Patellidas, with a long radula; teeth 2.4.2; muscular im- 

 pression on shell crescentic. The gills surround the head 

 (Patella), or may be interrupted there (Nacella). Lepetids 

 have no eyes nor gills. Tecturidae have a free gill on the 

 right side of the neck. 



§ III. Aspidobranchiata — herbivorous, rhipidoglossal ; 

 shell conical, spiral, flat or cap-like; foot large ; gills double; 

 penis none. This includes — i. Fissurellidae — foot large; 

 shells flat (Parmophorus) or limpet-like, with an apical (Fis- 

 surella) or pre-apical (Rimula) opening, or an anterior mar- 

 ginal fissure (Emarginula).* 2. Haliotidse — shells ear-like, 

 nacreous within, left border with a row of holes along the 

 suture; an anal siphon. 3. Pleurotomaridse — conical, fossil 

 shells, with a notched mouth, represented now by Anatomus 

 and Stomatia. 4. Trochidse — scutibranchiate ; foot lobate 

 and filamented along the side ; shell nacreous within ; oper- 

 culum smooth and calcareous in Phasianella ; orbicular, horny 

 in Turbo, with a solid convex, calcareous, inner pagina, 

 and a turbinate ventricose shell. Imperator has an oblong 

 or ovate operculum, also calcareous within ; in Adeorbis it is 

 calcareous, multispiral. Rotella has a flat, polished shell ; 

 Delphinula a horny concentric operculum and a turbinated 

 shell ; Trochus a membranous operculum and a conical shell 

 with a flat base. Gena is ear-shaped, inoperculate, and Bro- 

 deripia is limpet- like. 5. Neritidse — scutibranchiate ; foot 

 oblong, triangular, not fringed ; shell thick ; peritreme cres- 

 centic, spire not prominent ; operculum calcareous, pauci- 

 spiral, often articulate. Nerita is marine ; Neritina freshwater ; 



* This genus has a rudimental opeixuligerous lohe. 

 X 2 



