44 



THE VOYAGE OF H.M.S. CHALLENGER. 



Some groups such as the Gastropoda and the Lamellihranchiata among the Mollusca, 

 which are such abundant and characteristic elements of the faunae of the shore-belts, 

 are scarce and comparatively insignificant, whUe some families, of which we have 

 hitherto known only rare and stunted examples, chiefly from the Boreal province, show 

 a large number of conspicuous genera and species, and give a marked character to tlie 

 abyssal fauna, which is consequently very recognisable. The extent of the area which 



Fig. 21. — Forms of the Phaodaria. 



is covered by this fauna is enormous, but the number of genera and species which it 

 includes does not seem to be inexhaustible. After the first year of the Challenger's 

 voyage, during which the Atlantic had been crossed and recrossed, we had a very good 

 idea beforehand what would be the general appearance of the group of animals forming 

 the contents of the trawl or dredge. Through the Indian Ocean and down to the 

 Antarctic Circle, through the seas of Australia and Polynesia, and in the South and North 

 Pacific, the general character of the fauna was the same. The same species were often 

 repeated, but more frecpently, especially at Stations at great distances from one another, 

 the species were different, but were markedly representative ; that is to say, they bore to 

 one another a close genetic relation. 



Perhaps among the groups which, on account of their speciality, their abundance, 

 and their universal diffusion, may be reckoned the more characteristic of the abyssal 

 fauna, are an order of rhizopods closely allied to the true Eadiolaria, described by 

 Professor Hseckel under the name Phseodaria. To this order are referred, with others, 

 a family of extremely graceful forms hke highly ornamented and beautifully sculptured 

 Foraminifera with silicious tests, — the Challengeridse. The Phseodaria are free- 

 swimming, and inhabit the cold water of the abyssal area. The Hexactinellidae 

 (figs. 14, 15, 16), an order of sponges unknown in shallow water, are very abundant at 

 aU depths in the abyssal region. The HexactinelHdse are extremely common in the Chalk 

 and Greensand, represented chiefly by the Ventriculites, and occur not uncommonly, re- 



