GASTROPODA. 11 



spiny tongues. That of the common English Limpet {P. vulgata. 

 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 Rock-Limpets, which have to resist the fury 

 of the breaking Avaves. 



The "Keyhole Limpets" and "Slit Limpets" {Fissurellidce) Cases, 

 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 PupiXlma aperta 

 is also all but hidden beneath the mantle of the animal. 



Fig. 5.* 



Pleurotomaria adansoniana. Case 9. 

 J natural size. 



The Pleurotomarke. are extremely rare in recent times, only six Case 1). 

 species being known, whereas over a thousand fossil forms have 



* From ' The Cambridge Natural History,' Messrs. Maemillan & Co. 



