MARINE MOLLUSCA OF THE UNITED STATES. 57 



nacreous inside ; operculum calcareous and pauci-spiral, or horny 

 and multispiral. 



III. Shell ear or limpet shaped* scarcely spiral; aperture basal, very large. 



Family X. Janthinid^e. Shell spiral, turbiniform ; fragile ; 

 aperture large; outer lip notched in the middle; no operculum. 



Family XI. Fissurellid^e. Shell conical, limpet-shaped; apex 

 recurved ; nucleus spiral, often disappearing in the course of 

 growth ; anterior margin notched or apex perforated ; muscular 

 impression horse-shoe shaped, open in front. 



Family XII. Calyptr^eid^e. Shell limpet-like, with the apex 

 more or less spiral ; interior simple or divided by a shelly process, 

 variously shaped, to which the adductor muscles are attached. 



Family XIII. Patellid^e. Shell conical, limpet-shaped, the 

 apex entire and turned forwards; muscular impression horse-shoe 

 shaped, open in front. 



Family XV. Chitonid^e. Shell oval, composed of eight transverse 

 imbricating plates, lodged in a coriaceous mantle, which forms 

 an expanded margin round the bodj\ 



IV. Shell tubular, symmetrical, open at both ends ; not sjriral. 

 Family XIV. Dentaliad^e. (The tooth shells.) 



Family I. XATICID.E. 



The Xatices, although possessing shells with entire apertures, 



are carnivorous animals, feeding principally on small bivalves, 



boring small round holes through their shells and rasping out the 



meat. 



Synopsis of Genera. 



Shell sub-globose ; spire rather elevated ; aperture semilunar ; columella 

 adherent to and spirally produced into the umbilicus ; apex more or less 

 dilated and truncate ; more rarely convex or rounded ; operculum horny, 

 with a calcareous outer layer. Natica, Adanson. 



Shell oval, subglobose ; spire rather elevated ; aperture semilunar ; inner 

 lip thin or with a moderate callus ; umbilicus wide, pervious, not funicu- 

 late. Operculum simple, cartilaginous. Ltjnatia, Gray. 



Shell orbicular, depressed ; spire flattened ; aperture wide, semilunar ; inner 

 lip straight, callous ; columella adherent to and spirally contorted into 

 the umbilicus ; apex more or less dilated and truncate. Operculum simple, 

 cartilaginous. Neverita, Risso. 



Shell ovate or subovate, solid, smooth, usually without epidermis ; spire 

 small, acute, whorls simple ; aperture semicircular ; inner lip oblique, 

 thickened, callous ; umbilicus funiculate ; aperture adherent to and 



