6 THE VOYAGE OF H.M.S. CHALLENGER. 



forms of -whicli the specific claims have been definitely established. No permanent list 

 of either recent or fossil species can at present be tabulated ; but in order that the desired 

 result may ultimately be attainable, it is necessary from time to time to lay before the 

 public the progress that has been achieved in the right direction, pointing out at the 

 same time the unavoidable deficiencies in our knowledge. The ranges in depth recorded 

 in our list are even now sufficient to warrant us in arriving at certain general inductions. 

 Thus, for the sake of argument, we will put down the number of recorded species and 

 named varieties at nominally 135 — viz., 125 so-termed species, and 11 named varieties — 

 a number which will certainly have to be hereafter reduced. As nothing is known 

 respecting the ranges of depth of some 25 or 26 so-called species, of which we have given 

 a list, the number upon which we may venture to generalise would be about 107. 

 In approximate numbers we find — 



From shore to 500 fathoms, some ..... 98 species. 



Or named varieties ; 12 of these range up to 100 fathoms 

 or less. 



From 501 to 1000 fathoms, 16 „ 



Of these only one, Discina atlantica, would range from 

 690 to 2400 fathoms. 



From 1001 to 1500 fathoms, 6 „ 



Of these, Terebratula wi/vill/i ranges from 1035 to 2900 

 fathoms, the greatest depth at which any species has 

 been found. 



From 1501 to 2000 fathoms, 4 „ 



From 2001 to 2900 fathoms, ...... 3 „ 



Thus, out of 107 species or named varieties, some 57, or about half the known 

 species, were dredged at a depth of under 100 fathoms; 20 to 25 at low- water mark, 

 or from 5 to 10 fathoms; and the remainder at about 50 or 60 fathoms. These facts 

 indicate that the greater bulk of known species live at com|)aratively small or moderate 

 depths, few in depths ranging up to 500 fathoms, and that Brachiopoda are specifically 

 rare at depths varying from 500 to 2900 fathoms. It must, however, in fairness be 

 noted that the number of deep-sea dredgings is small when compared with those made 

 in seas of moderate depths ; and, consequently, that a proportionally larger number of 

 • species may be hereafter expected when a larger area of the oceanic abysses has been 

 explored. I do not, however, anticipate that the general results will much alter the 

 conclusions formulated in the preceding pages. It is also evident that some species 

 are capable of existing at a great variety of depth ; for instance — 



Platydia anomioides is recorded from 40 to 600 fathoms. 



Rhynclionella psittacea „ 10 to 690 „ 



Terebratula vitrea „ 5 to 1456 „ 



Discina atlantica ,, 600 to 2425 ,, 



Terebratula wyvillii „ 1035 to 2900 



