178 BULLETIN OF THE 



water on the coast of Florida by Pourtal^s, and by the " Blake " in the 

 West Indies, the form is more depressed, the shell far more delicate, 

 the colors pale pearly tints of lemon and pink. It seems as if difi'er- 

 ences of temperature and nutriment, as between the north and the 

 tropics, were indicated in very similar ways, both by the dwellers in 

 the deep sea and those which inhabit the laud. 



It might be thought that in the abysses, of whatever latitude, the 

 conditions would be so similar that we should find the same animal 

 presenting few, if any differences, from whatever part of the ocean it 

 might come. This is to some extent true of the great oceanic deeps 

 away from the continental shores and archipelagos. There the water 

 is always cold, and a certain and not very profuse mollusk fauna has 

 been found widely spread ; having apparently migrated from the polar 

 regions, and perhaps especially from the south polar regions, into the 

 deeps of both hemispheres. It is very necessary, in considering the dis- 

 tribution of the deep-sea mollusks, to bear in mind the different values 

 which the expression " deep sea " has had, and which, if confounded, 

 would give rise to serious errors. 



Formerly, when dredging with the usual appliances in small boats, 

 one hundred fathoms was considered extremely deep, and specimens 

 from even half that depth were considered as having come from deep 

 water. This was proper enough when the collections were compared 

 with those from the shore between tides, or even from the adjacent 

 region below tide-marks, but which supported a growth of algae, either 

 ordinary sea-weeds, or the solid calcareous kinds known as corallines. 

 But when naturalists began to investigate at much greater depths, the 

 old terms lost their meaning. 



For present purposes deep-sea mollusks may be taken to include all 

 those living at depths too great to allow algce of any sort to flourish, 

 the limit depending somewhat on the locality. Those living only above 

 that limit would form the littoral fauna, which, roughly speaking, may 

 be said to extend from the shores to about one hundred fathoms in 

 depth. With them in suitable places would be mixed many deep-water 

 forms, which extend their range to shallow water without being charac- 

 teristic of it. 



The remainder of the sea would naturally be divided rather by tem- 

 perature than depth. But the temperature itself is somewhat dependent 

 upon the depth, the influence of the great warm currents of the ocean 

 rarely extending below seven or eight hundred fathoms, and this depth 

 corresponds roughly to a temperature of about forty degrees Fahren- 



