FACTS AND OBSERVATIONS. 



xlix 



An inquiry into the nature and position of sea-grounds, conducted with considerable care, 

 results in the fact that every kind of ground, except rock, exists at one place or other at almost 

 all depths, small and great the nature of the successive depths being, in a rough way, that which 

 has been adopted in Table Z. The exceptions are numerous. The following are instances of the 

 greatest ; the others I must neglect. 



Blocks of stone at 200 fathoms (Greenland, Dr. Wallich). Shingle at 1675 fathoms (Capt. 

 Dayman). Gravel at 2330 fathoms (Atlantic Telegraph route, Dayman). Sand at 954 fathoms 

 (Atlantic Telegraph route, Dayman). Brown mud, clays principally, at 180 fathoms (.ZEgean Sea, 

 Forbes) . Weed; Dr. Wallich has met with no " Algae proper " below 200 fathoms. NuUipore at 130 

 fathoms (Algiers, Milne-Edwards) . Shelly ground at 54 fathoms. Coralline at 145 fathoms, at which 

 depth Mr. Gwyn Jeffreys finds Chiton cinereus and Trochus granulatus. White mud ; this ground in 

 an especial manner belongs to the deepest parts of the sea, but it is found at all levels in areas of 

 smaller sizes. 



The presence of this or that ground may usually be accounted for by local circumstances by 

 ocean depths, contours of and distance from land, by the constituents of the nearest coasts, by the 

 presence or absence of headlands, of great rivers, of steady, variable or conflicting currents, by pre- 

 vailing temperatures, and other well-known influences. These are all cosmic agencies. 



The causes of divergence, as they now occur to me, are the following : 



1. Currents (tides &c.) driving the fauna from their grounds. 



2. Changes of level, damaging or removing their grounds, rendering them in fact less 



desirable for shelter or pasture. 



3. Injurious changes in the nature of the faunal community ; the carnivorous mollusca 



have devoured all the herbivorous, or the latter all the plants. 



4. The free swimmers, Cephalopoda and Pteropoda, are dropped in a dead state into various 



grounds, because they live independently of all sediment. 



We now lay before the reader the Table Z. It exhibits, with a certain degree of accuracy, 

 the distribution of a large number of marine species of mollusks among the principal sea-grounds of 

 the present day, as seen in eleven large regions. 



TABLE Z. Molluscan Sea-grounds. 



(a) In twenty-three cases grounds are omitted by the authors. (6) In fifteen here also. 



* Eeport British Association, 1850. 



J Reports British Association, 1850 and 1856. 



|| Ibid. 1856. f Ibid. 1856. 



t British Conchology, 1863. 

 M'Andrew & Barrett, ibid. 

 ** Ibid. 1866. 



n 



