Limpets. 107 



rows, four deep, of sharp hooked teeth, and between 

 each row are placed two others, which have three 

 points, and are set in a slanting position. The use 

 of this arrangement we cannot at present deter- 

 mine, but no doubt it has a perfect adaptation to 

 the wants of the animal. 



There are shell-fish called Key-hole Limpets, 

 which belong to the genus Fissurellidce, from jissura, 

 a cleft or slip, from whence comes also fissure. All 

 the members of this genus are distinguished by the 

 aperture at the top of the shell, shaped like a key- 

 hole, which is situated exactly over the breathing 

 organs, and serves as a channel for the water neces- 

 sary for respiration. 



Frequently upon the fronds of the large olive 

 sea-weeds may be found a tiny shell shaped some- 

 thing like that of the Common Limpet. It is of an 

 olive-green colour, with blue streaks, and is called, 

 from its clearness, the Pellucid Limpet (P.pellucida). 

 There is also another much like it in appearance, 

 which naturalists call P. Icevis. To the labours of 

 these little mollusks, according to Dr. Harvey, may 

 be attributed the destruction of the gigantic Alga3 

 (sea- weed). Eating into the lower part of the 

 stems, and destroying the branches of the roots, 

 they so far weaken the base that it becomes unable 



