520 MOLLUSCA phtlüm vi 



Shells themselves exhibit little Variation in form, and hence tlieir generic and even 

 family affinities are almost always doubtful in tlie fossil state. About 400 Eecent 

 species of limpets are known ; these are almost exclusively shallow water inhabitants, 

 and subsist on algae. Fossil forms are uncommon. 



In tlris very primitive groiip two divisions have been proposed by Ulrich and 

 Scofield : (1) the Patellacea, which embraces the first tliree families noted below ; and 

 (2) the Bellerophontacea, including the remaining five families. The latter group by 

 Meek was regarded as involute Fissureil idae, a view which is not withoiit plausibility. 



Family 1. Patellidae Carpenter. 



Patella Linn. Cup-shaped, round or oval, depressed conical, with sub-central or 

 eccentric apex. Surface usually with radiating ribs or Striae. Silurian to Eecent. 



Helcion Montf. Differs in having the beak strongly recurved anteriorly. Eocene 

 to Eecent. 



Helcioniscus Dali ; Nacella Schum. Eecent. 



Family 2. Acmaeidae Dali. 



Acmaea Eschscholtz {Tectura auct.) (Fig. 844, B). Like Patella, but shell having 

 generally a differentiated marginal band inside ; externally smooth, finely Stria ted, or 

 radially ribbed. Beak anterior to the middle. Silurian to Eecent. Lottia Gray is 

 closely allied. 



Scuiria Gray (Fig. 844, G). High conical, smooth, with sub-central beak. Jura 

 to Eecent. 



A B C D 



844. 



A, Archinacella cingulata Vir. Ordovician ; Kentucky. B, Acmaea raincourti Deah. Eocene; Auvers, near 

 Paris. C, Scurria nitida Deslongcli. Upper Jura ; Langrune, Calvados, l/i. D, Helcionopsis striata Ulr. 

 Ordovician ; Kentucky. 



Metoptoma Phil. Depressed conical with sub-central beak. Posterior side exca- 

 vated. Silurian to Carboniferous. 



Lepetopsis Whitf. Silurian to Carboniferous. 



The genera, Palaeacmaea Hall; Archinacella Ulrich and Scofield (Fig. 844,^); and 

 Scenella Billings are the oldest representatives of the Docoglossa. They are small, 

 smooth or radially ornamented, and scarcely to be distinguished from Acmaea. 

 Lepeta Gray and Lepetella Verrill are small simple limpets of the Eecent and late 

 Tertiary, with degenerate, aborted gills. They form the family Lepetidae. * 



Helcionopsis Ulr. and Scof. (Fig. 844, i>), and Gonchopeltis Walcott, from the 

 Ordovician of North America, are doubtfully referred to this vicinity. 



Family 3. Tryblidiidae Pilsbry. 

 Limpets with the muscle scar broken into numerous separate impressions. Silurian. 



