GENERAL INTRODUCTION TO THE ZOOLOGICAL REPORTS, 43 



volcanic minerals. In this view I am now inclined to agree with him, though I still 

 believe that a large amount of the molecular inorganic matter which is abundant in the 

 red clay deposits must be due to the silicates of alumina and iron which we know exist 

 in appreciable quantity both in the hard and soft parts of marine animals. The greatest 

 extent of the " red clay " area is probably in the Pacific, where the average depth of 

 the ocean basin is considerably greater than it is in the Atlantic or the Indian Ocean. 



The red clay is not so favourable to the development of animal life as the globigerina- 

 ooze, and a large proportion of the animals living on a red clay bottom belong to groups 

 which secrete but little calcic carbonate in their tests or other hard parts, such as the 

 arenaceous Foraminifera, Hexactinellid Sponges, Holothurideans, and tube-forming 

 Annelids. 



The Uniformity of the Abyssal Fauna. — The Challenger during her long 

 cruise passed over an extended area under very varied surface conditions. From 

 the circumstances of her voyage it was impossible to examine any one locality fully, 

 but enough was done to enable us to gain a sufficient idea of the general distribution 

 of the more conspicuous animals living on the bottom of the sea, to justify the con- 



FiG. 20.~Aerope rostrata, Wy. T. One of the abyssal irregular urchins. 



elusion that, at depths below 500 to 600 fathoms, a fauna exists of extreme uniformity, 

 which it is impossible as yet to break up into regions or provinces on zoological 

 grounds. Apparently all the classes and most of the leading orders of marine 

 invertebrata are fuUy represented, but their representation is not in the same relative 

 proportions as in the lesser depths with which we have been hitherto acquainted. 



h 



