SUBMARINE DEPOSITS 187 



lumps and broken fragments, and water-worn particles 

 of the branched nullipore Lithoihamnion polymorphum 

 (Plate XI, Fig. 1) ; in others there may be deposits 

 almost wholly composed of the dead and broken shells 

 of lameUibranchiate mollusca, such as mussels, cockles, 

 and their aUies ; and on a bank off the south end of the 

 Isle of Man, at a depth of 20 fathoms, there is a white shell- 

 sand (Plate XI, Fig. 2) composed of broken fragments of the 

 mollusca Pec^en, Anomia, Pectunculus, Mactray Venus, Mytilus, 

 Cyprcea, Buccinum, Emarginula, Purpura, and Trochus, of 

 various calcareous Polyzoa, such as Cellaria fistulosa, 

 Cellepora pumicosa, and many Lepralids, of plates of 

 Balanus and tubes of Serpula, and of plates and spines of 

 several Echinoderms. Such a neritic deposit as this would 

 form a rock almost wholly made up of fossils, and might be 

 compared with some Tertiary deposits, such as the Coralline 

 and Red Crag formations of Suffolk. In one of the neritic 

 deposits south of the Isle of Man, close on sixty species of 

 Polyzoa were recorded from one haul of the dredge.^ 



Although the neritic deposits are chiefly found on the 

 continental shelf near land, they may also occur in shallow 

 water on a submarine bank in the open ocean, surrounded 

 by deep waters with their characteristic pelagic oozes. It 

 may be argued that coral sands and muds are also neritic 

 deposits, as they are formed of the remains of the hard 

 parts of shallow- water organisms more or less in situ. But 

 if the coral reef (which may be a large, inhabited island) 

 be regarded as land, then the deposit derived from it may 

 be called " terrigenous." As Murray has pointed out, hard- 

 and-fast lines cannot always be drawn between some of the 

 categories of deposits ; they merge one into another by 

 insensible gradations, as is only to be expected when we 

 consider their mode of occurrence and origin. 



III. Pelagic (or Planktonic). — With the exception of the 



1 See Herdman and Dawson, Fishes and Fisheries of the Irish 

 Sea. London, G. Philip & Son, 1902. 



