1882.] 



On the Formation of Hipplemark. 



11 



afforded in the present paper. A very cursory glance at it mnst 

 suffice. 



The character of the fauna of the littoral zone is snch that it can 

 be seen at a glance that the chief enemy that has to be contended 

 against is the wash of the waves. Molluscs and crnstaceans living" on 

 rocks are specially adapted to cling tightly to those rocks, whereas 

 those living on sand have the power of burrowing in the sand. 

 Where the ground is solid, as in the case of rocks, the animals living 

 on it trust to their powers of holding on or of boring into it ; where 

 the gronnd is nnstable the animals that frequent it trust to their 

 powers of rapidly shifting their positions. Of the former class the 

 limpet is a good example ; of the latter the common razor fish. But 

 if the mollusca of the littoral zone are specially adapted to resist the 

 wash of the waves that would drive them high and dry on shore, it is 

 equally true that many of those living in the laminarian and coralline 

 zones are wonderfully provided against their special danger, viz., the 

 alternate swing of the waves on the bottom. Living as they do con- 

 tinuously nnder water, their shells are free to assume the most 

 elaborate sculpture and form, from which the littoral shells are pre- 

 cluded, owing to a compulsory cessation of growth twice a day by 

 retreat of the tide. In many cases the development of the lip, or of 

 the sculpture in the form of spines, supplies exactly what the animal 

 wants, viz., a broad base for a sandy bottom. By the kindness of the 

 Rev. A. Cook I have been able to experiment with a few winged and 

 spined shells from different parts of the world. One experiment 

 with a Murex monodon from Australia, a Pteroceras lambis from the 

 East Indies, a Strombus tricornis from the Bed Sea, and four speci- 

 mens of Aporrhais pes pelecani from Torbay was very instructive. 

 Placing them all on their backs in my tank, I succeeded on one 

 occasion in fifteen seconds in restoring them all to their proper posi- 

 tions, simply by swinging the water in the tank. Owing to the 

 weight of the foreign shells some difficulty was experienced in getting 

 them in motion, and moreover, with the exception of the Aporrhais 

 and Pteroceras, they were not particularly suitable for the experiment, 

 as they had not wings or spines very largely developed. 



In the case of the extremely long spined murex, M. tenuispina, 

 though the spines offer great resistance to the animal being overturned, 

 they do not afford any assistance to the animal to recover its balance 

 when once it has lost it. In the best examples of winged and spinous 

 shells, such as Aporrhais and Pteroceras, the alternate current requisite 

 to npset them is very mnch more powerful than one sufficient to 

 restore them to their normal position. 



As Aporrhais pes pelecani is a beautiful instance of a gasteropod 

 proof against moderate wave action, so the common Pecten maximus is 

 a good example of like protection among the bivalves. Owing to one 



