Handbook of Paleontology 97 



It has been observed that Limpets that live on exposed 

 flat surfaces have a shell that is typically low and 

 broad; but the same species on more sheltered surfaces 

 develop shells which are typically high and narrow. The 

 common Dog Whelk or Rock Snail {Purpura lapillus) 

 has also been studied along these lines. In very ex- 

 posed places the individuals have stunted shells, with a 

 short spire, and they have developed a large mouth to 

 increase their power of adhesion. In sheltered places, 

 such as estuaries and narrow straits, the individuals grow 

 to a comparatively large size; the spire is always well 

 developed, sometimes even produced; and the mouth is 

 small in proportion to the size of the shell. 



Animals living on sand or other unstable deposit have 

 an even more difficult problem to meet than those living 

 on rock or other firm foundation. With such an unstable 

 habitat no powers of attachment are of any service and 

 the safety of the animals therefore depends mostly upon 

 speed or ability to burrow. Starfishes, such as the com- 

 mon Asterias, lie rigid on the bottom with their arms 

 extended, which minimizes friction, and they also have 

 the power of sinking vertically into the sand. Swimming 

 crabs escape beneath the surface by very rapid shovelling 

 with the swimming feet. The Sand-eels that live along 

 sandy shores between high and low tide marks also burrow 

 rapidly beneath the surface to escape the action of the 

 waves. There are a number of snails, such as the Moon 

 Shells (Natica), Whelks (Buccinum), Nassas, Bubble 

 Shells (Bulla) etc. living on the sea floor which have no 

 special defenses against the action of the waves, but they 

 all burrow. Indeed on sandy and muddy bottoms bur- 

 rowing is the outstanding method of gaining protection, 

 and mollusks of the sand are mostly burrowers. Some 



