442, MOLLUSCA 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. 
Acmaed, Eschscholtz (Tectura, auct., Fig. 795). Like Patella, 
but shell having generally a differentiated 
Fic. 795. marginal band inside; externally smooth, 
Acmaew Raincourti, tinely striated, or radially ribbed. Beak 
eet ne} Auvers, 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, Whitt. 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 Me 
scarcely to be distinguished from Aecmaea. Lepeta, Gray, and ,,ccurr@ (mina Des: 
Lepetella, Verrill, are small simple limpets of the Recent and Langrune, Calvados. 1). 
late Tertiary, with degenerate, aborted gills. 


Fic. 796. 
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, Ihering.) 
Symmetrical and limpet-like, or with spiral 
shells. Gills plume-like, two and symmetrical, 
or single. Radula with several large plates or 
si, ; ; . see teeth in the median portion, and excessively 
Tryblidiwm vreticulatum, Lindstrom. Silurian ; } re 
Gottland. A, Internal, and B, external aspect numerous, crowded, narrow, hook-shaped mar- 
(after Lindstrém). ginal teeth. Operculum often present. 

Fic. 797. 
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. 
