442 



MOLLUSC A 



SUB-KINGDOM 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 1400 recent 

 limpets are known ; these are almost exclusively shallow water inhabitants, and sub- 

 sist on algae. Fossil forms are uncommon. 



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. 

 Acmaea, Eschscholtz (Tectura, auct, Fig. 795). Like Patella, 

 but 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. 796). High conical, smooth, with sub- 

 central beak. Jura to Recent. 



Metoptoma, Phil. Depressed conical, with sub-central beak. 

 Posterior side excavated. Silurian to Carboniferous. 

 Lepetopsis, Whitf. Silurian to Carboniferous. 

 The genera Palaeacmaea, Hall, from the Silurian, and Scenella, 

 Billings, from the Cambrian, 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 Recent and 

 late Tertiary, with degenerate, aborted gills. 



FIG. 795. ' 



Acmaea Raincourti, 

 Desh. Eocene ; Auvers, 

 near Paris. 



nitida, Des- 

 .__. Great Oolite; 

 Langrune, Calvados. l/i . 



Family 2. Tryblidiidae. Pilsbry. 



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



Tryblidium, Lindstrom (Fig. 797). Shell depressed, very thick, oval, with anterior 



beak ; ornamented externally with concentric 

 lamellae. Six pairs of muscle scars arranged 

 in the form of a horse-shoe. Silurian. 



Sub-Order B. RHIPIDOGLOSSA. 



Troschel. 



(Scutibranchiata, Cuvier ; Zygobranchia, 



Ihering; Diotocardia, Iheriiig.) 

 Symmetrical and limpet-like, or with spiral 

 shells. Gills plume-like, two and symmetrical, 

 or single. Eadula with several large plates or 



teeth in the median portion, and excessively 



TryUidium reticulatum, Lindstrom. Silurian; , ,7, -h finp ;i ninr 



Gottiand. A, internal, and B, external aspect numerous, crowded, narrow, hook-shapea - 

 (after Lindstrom). \] 



gina i ted ^ Qperculum often present. 



rrv ' The Rhipidoglossa comprise both air-breathing and aquatic forms, and are divisible 

 into two series: Zygobranchia, in which two gills are developed, and the shell is 

 generally perforated at the apex or has a slit in the outer lip ; and Anisobranchia, 

 with a single gill and generally unslit shell. 



