520 



MOLLUSCA 



PHYLUM VI 



shells themselves exhibit little variation in form, and hence their 



generic and 



even 



family affinities are almost always doubtful in the fossil state. About 400 Recent 

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

 and subsist on algae. Fossil forms are uncommon. 



In this very primitive group two divisions have been proposed by Ulrich and 

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

 (2) the Belleropho7itacea, including the remaining five families. The latter groujj by 

 Meek was regarded as involute Fissurellidae, a view which is not without 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 Recent. 



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

 to Recent. 



Helcioniscus Dall ; Nacella Schum. Recent. 



Family 2. Acmaeidae Dall. 



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

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

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

 closely allied. 



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

 to Recent. 



A BCD 



A, Ai-chinac.ellaciwjidataVlT. Ordovician ; Kentucky. B, Aciiuwa TainvourUV>6s\\. Eocene; Anvers, near 

 Paris. C, l$curria nitida Deslongcli. Upper Jura ; Langrune, Calvados, i/j. 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 Bocoglossa. 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 Recent and late 

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



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

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



Family 3. Tryblidiidae Pilsbry. 

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



