CIV 



INTRODUCTION. [CH. 



the shore, strand, or beach depends on the composition 

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

 opposed to tlie sea. The rocks lying between tide- 

 marks are clothed with seaweed, which supports a 

 numerons and peculiar group of Mollusca. Among 

 those on our own coasts may be enumerated various 

 species of Littorina (or periwinkle), Lacuna, Trochas, 

 Rissoa, Chiton, Patella (or limpet) , Purjmra lapillus (or 

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

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

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

 Una officinalis and other small seaweeds, as well as under 

 loose stones, will be found many small shells of various 

 sorts, including Poronia rubra, Modiola discors, Skenea 

 planorbis, Rissoa parva, Cerithium reticulatum, 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 [M. bidentatus and a variety of M. myosotis), 

 Otina Otis, Assiminia littorea, Truncatella truncatula, 

 a variety of the common limpet, Littorina Neritoides, 

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

 first three of the al)ove species are Pulmonobranch. 

 That part of the littoral zone which consists of sand, 

 gi'avel, or mud is frequented by various genera of bi- 

 valve ^lollusca, such as Mya, Solen, Tellina, Donax, 

 Mactra and Tapes, as well as by Mytilus edulis. Within 

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

 sandstone, harbour several kinds of Pholas ; Scrobicu- 

 laria piper ata burrows into clay ; calcareous rocks are 

 perforated by Saxicava rugosa ; and fixed wood is di^illed 

 in every direction by the destructive Teredines or ship- 

 worms. Wherever a river or stream empties itself into 

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



