Fissurellinae

The subfamily Fissurellinae, common name the keyhole limpets and slit limpets, is a taxonomic subfamily of limpet-like sea snails, marine gastropod molluscs in the family Fissurellidae.
Of the three subfamilies in the Fissurellidae, the subfamily Fissurellinae is the most recently evolved; its earliest known species date back to the Cenozoic.
The following genera in this subfamily have characteristically a body that tends to be larger than the shell.

AmblychilepasDendrofissurellaMedusafissurella

They have sometimes been confused with genera in the emarginuline tribe Fissurellideini.
Other genera include : Altrix - Clathrosepta - Cornisepta - Cosmetalepas - Dendrofissurella - Elegidion - Fissurella - Laevinesta - Lucapina - Macroschisma - Manganesepta - Montfortula - Montfortulana - Parmaphoridea - Parmophoridea - Profundisepta
The characteristics that distinguish them from the subfamily Emarginulidae are the following :

the rachidian plate of the radula has a broad base and a narrow tip
the outer lateral tooth in the radula is so long that it is aligned with the lateral teeth from the row above
the shell muscle lacks an inwardly directed hook-shaped process.

A molecular phylogeny however has shown that Fissurellinae is paraphyletic with respect to Emarginulinae. This is somewhat surprising as the subfamily Emarginulinae is generally accepted to be basal to the subfamily Fissurellinae