1878.] 



Mr. T. Davidson. On Brachiopoda. 



435 



1870 and 1873 by Mr. W. H. Dall. I have in the above list given all 

 the ranges of depth at present known to me ; but of twenty-six species 

 no information is given by the original authors, and later research has 

 not revealed it. It is to be hoped that future dredging expeditions 

 will supply the desiderata. It may also be stated that some twenty 

 of the recent species have also been found in the upper tertiary forma- 

 tions. No permanent list of the recent species can at present be tabu- 

 lated ; but in order that the desired result may ultimately be attain- 

 able, 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 some 25 or 26 so called 

 species, and which are given in the list, the number upon which we 

 may venture to generalize would be about 107. In approximate 

 numbers we find, 



From shore to 500 fathoms, some 97 species. 



Or named varieties, 12 of these range up to 



1,000 fathoms or less. 

 From 501 to 1,000 fathoms 16 „ 



Of these only one, Discina Atlantica, would 



range from 690 to 2,400 fathoms. 

 From 1,001 to 1,500 fathoms 6 „ 



Of these Ter. Wyvilli ranges from 1,035 to 



2,600 fathoms, the greatest depth at which 



any species has been found. 



From 2,501 to 2,000 fathoms 3 4 



From 2,001 to 2,600 fathoms 3 



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

 the known species, were dredged at depths of about 100 fathoms ; 

 20 to 25 at low water mark, or from 5 to 10 fathoms ; the larger 

 number up to 50 or 60 fathoms. These facts indicate that the greater 

 bulk of known species preferred to live at comparatively small or 

 moderate depths, but few in depths ranging up to 500 fathoms, and 

 that Brachiopoda are specifically rare at depths varying from 500 to 

 2,600 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 proportionably larger 

 number of species may be hereafter expected when a larger area of 

 oceanic abysses have been explored. I do not, however, anticipate 

 that the general results will much alter the conclusions formulated in 



