GASTROPODA. 11 
spiny tongues. That of the common English Limpet {P. vulgata y 
Fig. 3) is longer than the shell itself, and armed with as many as 
1920 glassy hooks in 160 rows of twelve teeth each. The Limpet 
is commonly used for bait in the sea-fishing off the Scottish coast, 
and vast quantities are consumed as food in some parts of Ireland. 
Some Limpets, such as P. compressa, P. mytilina, etc., are found on 
the stems of floating seaweeds, and have the shells usually thinner 
and smoother than the Bock-Limpets, which have to resist the fury 
of the breaking waves. 
The "Keyhole Limpets" and "Slit Limpets" (Fissurellidce) Case 8. 
resemble in external shape ordinary Limpets, but are perforated at 
or near the apex, or more or less slit at the front margin. The 
hole or slit gives passage to a tubular fold of the mantle, through 
which the water apparently flows to the gills. The largest species 
are from California and South America, and others are found, but 
not abundantly, on most shores. The animal of the large Lucupina 
crenulata from California is eight or ten inches in length, and almost 
conceals the shell, and the shell of the South- African Pupillwa aperta 
is also all but hidden beneath the mantle of the animal. 
Fig. 5.* 
Pleurotomaria adansoniana. Case 9. 
J natural size. 
The Pleurotomaria are extremely rare in recent times, only five Case 9. 
species being known, whereas over a thousand fossil forms have 
* From ' The Cambridge Natural History,' Messrs. Macmillan & Co. 
