Civ TXTROWCTION. [CH. 



the shore, strand, or beach depends on the composition 

 of the strata which form that part of the land which is 

 opposed to the sea. The rocks lying between tide- 

 marks are clothed with seaweed, which supports a 

 numerous and peculiar group of Mollusca. Among 

 those on our own coasts may be enumerated various 

 species of Littorina (or periwinkle), Lacuna, Trochun. 

 Rissoa, Chiton, Patella (or limpet), Purpura lapiUus (or 

 dogs'-whelk), and a stunted variety or form of the com- 

 mon mussel. In the small rock-pools, which are left 

 by the receding tide, and are generally lined with Coral- 

 lina officinal is and other small seaweeds, as well as under 

 loose stones, will be found many small shells of various 

 sorts, including Poronia rubra, J[odiola discors, Skenea 

 planorbis, litssoa parra, Cerithhtm rctieulatum, and the 

 fry of other species. The highest part of this zone, 

 which the sea does not cover for more than two or three 

 hours out of every twelve, is inhabited by two kinds of 

 Melampus (J/. bidentatus and a variety of M. mi/o$oti$\ 

 Otiua otis, Assiminia Httorea, Truncatella truncatuht, 

 a variety of the common limpet, Littorina Neritoides, 

 and some of the numerous varieties of L. rudis. The 

 first three of the above species are Pulmonobraneh. 

 That part of the littoral zone which consists of sand, 

 gravel, or mud is frequented by various genera of bi- 

 valve Mollusca, such as J///<?, Solcn, TelUna, Domt.r, 

 M<ictra and 7 <?/><>, as well as by Mytilns ed-u!i$. Within 

 this zone submarine peat, chalk, and trias or new red 

 sandstone, harbour several kinds of Pholas ; Scrobicu- 

 laria pipcrata burrows into clay ; calcareous rocks are 

 perforated by Sa.fi cava rngosa- ; and fixed wood is drilled 

 in every direction by the destructive Teredines or ship- 

 wonns, Wherever a river or stream empties itself into 

 the sea, a strong reflux is caused by the advancing tide, 



