Hi Harry,

"Limpets" is a very general term, which describes a particular form of shell,
without regard for whether of not the various mollusks having that shell form
are closely related.  As a result, several non-related families of mollusks,
with superficially similar shells, are called "limpets".  These include:

Patellidae - "true" limpets
Acmaeidae - "Acmaea" limpets, sometimes included under "true" limpets
Lottiidae - sometimes called "owl limpets", though that name really only
belongs to one species of the family
Fissurellidae - keyhole limpets, slot limpets, and fleshy limpets
Calyptraeidae - cup & saucer limpets and slipper limpets
Capulidae - cap limpets
Hipponicidae - hairy limpets or hoof limpets
Siphonariidae - siphon limpets or false limpets
Lepetidae - no common name that I know of
Acroloxidae - fresh water limpets

Paul M.
Rhode Island