388 



LIMPETS. GEOLOGICAL DISTRIBUTION. 



FIG. 584. CHITON. 



which endeavour to detach them for food, are sometimes caught 

 by the points of their bills, and are held there until they are 

 drowned by the advancing tide. The Limpets are herbi- 

 vorous, feeding upon sea-weeds, which 

 they reduce with their long riband-shaped 

 rasp-like tongues. Although we usually 

 see them attached quite motionless to 

 rocks, this is because exposure to the air 

 is unfavourable to their movement, since 

 too free an admission of it between their 

 gills would dry them up. It is when 

 covered with water, that their activity 

 manifests itself. In many points of struc- 

 ture, the animals of this Order approach 

 those of Bivalve shells. The rock, in the 

 Limpet, may almost be regarded as a second 

 valve ; since the muscle takes a firm 

 attachment to its surface, and draws down the shell upon it, in 

 the same manner as the adductor muscle of the Bivalves enables 

 the animal to inclose itself by drawing the two valves of the shell 

 together ( 932). Moreover the eyes are very imperfect or entirely 

 absent ; and many other points of internal structure prove the 

 inferiority of these animals to the Gasteropods in general. 



929. To enter into any detailed account of the Fossil Remains 

 of this class, would be unsuitable to our present purpose ; and it 

 will be sufficient here to state some general facts in regard to 

 them. Remains of Univalve shells, of such a form and struc- 

 ture as evidently to have belonged to Gasteropod Mollusks, are 

 found in nearly every bed formed by the action of water, from 

 the very earliest of those containing fossils of any kind, down to 

 the present time. In many instances, the fossil shells, even of 

 the most ancient beds, may be referred to genera which still 

 exist ; thus a species of Buccinum is found in the rocks of the 

 Silurian system ; although they do not correspond with any 

 species now living, except in cases where there is reason to believe 

 that the deposit was formed at a comparatively recent period. 



