110 GBOGEAPHICAL DISTRIBUTION. 



of some two thousand five hundred fathoms shell accumulations 

 almost completely cease, their place being taken by a "red-clay" 

 deposit, whose exact nature is, perhaps, not yet clearly understood. 

 The absence of foraminiferal tests from the areas of greatest depth 

 is doubtless due to the dissolution of the calcareous matter of the 

 shell during its descent from the surface to the bottom, most of the 

 pelagic forms, there is good reason to believe, being restricted in 

 their range to a superficial zone of a few hundred fathoms depth. 



The sponges, although they attain a maximum development in 

 a zone of five hundred to one thousand fathoms, extend to the 

 greatest measured depths, and are represented in the deepest parts 

 by all the recognised orders, with the exception of the Calcarea 

 (calcareous sponges), which appear to be confined to shallow water. 

 The HexactinellidsB, among siliceous sponges, to which the "glass- 

 rope sponge " (Hyalonema) and Venus' flower-basket (Euplectella) 

 belong, and whose earliest representatives appear already in the 

 fauna of the Cambrian period, preponderate in the abyssal regions. 

 The ordinary homy sponges (Keratosa), while they possess a very 

 considerable vertical range, have their special development in the 

 coralline zone. In the deepest parts of the sea corals are but spar- 

 ingly distributed, and by far the greater number of species belong 

 to the type of simple corals, and to the family Turbinolidaa, a large 

 proportion of the genera passing back to the Tertiary period, and a 

 few to the Cretaceous. No true Paleozoic forms have as yet been 

 discovered. But five genera of Madreporaria are known whose 

 range extends to, or exceeds, fifteen hundred fathoms (nine thou- 

 sand feet), and only a single one, Bathyactis, which transgresses the 

 twenty-five hundred fathom line. The vertical range of some of 

 the species is very extraordinary, most notably so in the case of 

 Bathyactis symmetrica, which is found in all depths between thirty 

 (Bermuda) and twenty-nine hundred fathoms (east of Japan).-' 



Several species of Medusse have been obtained from depths re- 

 ported to exceed two thousand fathoms; but it is, perhaps, open 

 to question whether some, or even most, of these apparently deep- 

 sea forms are not in reality inhabitants of a comparatively shallow 

 superficial zone, and have not been simply caught in the hauling of 

 the net. There appears to be strong evidence, however, for con- 

 cluding that at least a few of the forms are actually inhabitants of 

 deep water. 



