Besides giving the lists and range in depth of the Mollusca and Radiata, the Report entered into the question of their distribution considered in its bearing on geology. The observations of Prof. Forbes ranged over a period of eighteen months ; and his lists are based on more than 100 fully recorded dredging-operations in various depths from 600 to 780 ft., besides numerous coast-observations. The result of this valuable and special inquiry was to determine more clearly than had hitherto been done the range of species in depth, and the division into zones first proposed by Risso. With regard to much that Prof. Forbes accomplished no question has arisen. Of the eight zones into which he divides the bathymetrical distribution of the Mollusca, the first seven, ranging from the surface to the depth of 630 ft., although possibly too much subdivided, may be applicable to other seas where the conditions are similar ; but with respect to his eighth region, which extends from 630 to 1380 ft., Prof. Forbes's generalizations, although correct within certain areas, have been found inapplicable to the two great oceans and applicable only to parts of the Mediterranean. He observes, speaking of this eighth zone, " throughout this great and, I may say, hitherto unknown province, we find an uniform and well-characterized fauna ;" but then he goes on to say, " within itself the number of species and of individuals diminishes as we descend, pointing to a zero in the distribution of animal life as yet unvisited." He placed this zero at about 1800 ft.
In a subsequent work *, however, in speaking of the eighth region of depth, E. Forbes remarks, " its confines are yet undetermined, and it is in the exploration of this vast deep-sea region that the finest field for submarine discovery yet remains." " In the Mediterranean, as might be expected, when we consider the peculiar condition under which that great land-locked basin is placed, there are peculiarities in the distribution of both animal and vegetable life which require special consideration ;" and in speaking of animal life in the " Arctic province " of the Atlantic, he notices that the Mollusca appear to range much deeper in high latitudes than they do in more favourable climates, and mentions the capital haul made by Mr. Harry Goodsir in Davis's Straits, when a variety of shells, Crustacea, Echinoderms, and Corallines were brought up from a depth of 1800 ft.
In another Report†, on British Marine Zoology, E. Forbes divided the range of the Mollusca into only four zones of depth ; and
- Natural History of European Seas, p. 27.
† Brit. Assoc. Rep. 1850.
