GASTEROPODA. 277 
Type, OC. fornicata, Pl. XI., Fig. 16. 
Synonym, Crypta, Humph. 
Shell oval, limpet-like; with a posterior, oblique, marginal 
apex; interior polished, with a shelly partition covering its 
posterior half. 
The crepidule resemble the fresh-water navicelle in form ; 
but the internal ledge which mimics the columella of the nerite, 
is here the basis of the adductor muscles. 
They are sedentary on stones and shells, in shallow water, and 
are sometimes found adhering to one another in groups of many 
successive generations. The specimens or species which live 
inside empty spiral shells are very thin, nearly flat, and 
colourless. 
Distribution, 54 species. West Indies, Honduras, Mediter- 
ranean, West Africa, Cape, India, Australia, West America. 
Fossil, 14 species. Hocene—. France, North America, and 
Patagonia. 
PILEOPSIS, Lam. Bonnet-limpet. 
Etymology, pileos, a cap, and opsis, like. 
Synonyms, Capulus, Montf. Brocchia, Bronn. 
Type, P. hungaricus, Pl. XI., Fig. 17, P. militaris, Pl. X1., 
Fig. 18. 
Shell conical; apex posterior, spirally recurved; aperture 
rounded ; muscular impression horse-shoe shaped. 
Animal with a fringed mantle-margin; lingual teeth like 
calyptrea. 
P. hungaricus (the Hungarian-bonnet) is found on oysters in 
5 to 15 fathoms water; more rarely as deep as 80 fathoms, and 
then yery small. P. militaris is extremely like a velutina. 
Distribution, 8 species. West Indies, Norway, Britain, 
Mediterranean, India, Australia, California. 
Fossil, 20 species. Lias—. Hurope. 
Sub-genus. Amathina, Gray. A. tricarinata, Pl. XI., Fig. 19. 
Shell depressed, oblong; apex posterior, not spiral, with 
three strong ribs diverging from it to the anterior margin. 
Platyceras, Conrad (acroculia, Phil.). P. yetustus. Carb., 
limestone. Britain. 
Fossil, 20 species. Deyonian—Trias. America, Europe. 
Metoptoma, Phillips. M. Pileus, Ph. 
Shell limpet-like, side beneath the apex truncated, resembling 
the posterior yalye of a chiton. 7 species. Carb. limestone. 
Britain. 
