BATHYMETRIOAL DISTRIBUTION OP MARINE MOLLUSKS. 175 



Josephine (1869), Voringen (1876); French, Travailleur {l^^Q). 

 JS'o greater depth than about 4000 fathoms has been obtained, 

 and the average depth appears to be from 1500 to 1800 fathoms. 

 At the great depth of 2300 fathoms, the naturalists of the Porcu- 

 pine obtained the following mollusks, including their soft parts: 

 Si/ndosmya nitida^ Dacriidium lyitreum^ Fecfen fenestratiis, Nedera 

 ohesa^ Deatalium candidum. 



The water of the ocean gradually becomes cooler from the 

 surface to the bottom. The cooling is rapid between the surface 

 and 250 fathoms, slow but regular between 250 and 850 fathoms, 

 and almost without variation between 850 and 2300 fathoms. 

 The following temperatures were obtained by the Porcupine at the 

 mouth of the Gulf of Gascony, July 22, 1869. 



Surface, 17° Centio-rade ; 82 French metres,* 12^ Centigrade; 

 187 metres, 11° C; 402 metres, 10° C; 731 metres, 9° C; 1005 

 metres, 8° C, 1188 metres, 7° C; 1280 metres, 6° C; 1462 

 metres, 5° C; 1645 metres, 4° C; 2560 metres, 3° C; 4452 

 metres, bottom, 2'5° C. 



Three of the five vertical zones of marine life are contained 

 within the depths of surface to 50 fathoms, where the tempera- 

 ture decreases rapidly, and this change of temperature has, of 

 course, marked influence upon all animal life, both directly and 

 by governing the distribution of the plants upon which animals 

 largely feed. Whilst the surface temperature varies greatly in 

 different portions of the globe, the greater the depth the more 

 tendency is there to uniformity, until finally, there is a body of 

 water having the vast thickness of 1000 to 1500 fathoms, at 

 the depth of from 1000 to 2500 fathoms, with scarcel}^ any 

 variation in temperature. It is evident that those mollusks 

 which are able to live in any portion of this belt, may be expected 

 also at its greatest depth, and that in proportion to the depth at 

 which they live, their geographical distribution may increase. 

 Another element which deserves notice is that a shore species 

 in a northern latitude, may spread southwards retaining the 

 temperature suitable to its existence, by seeking greater depths. 

 These general facts are modified in particular localities, and the 

 distribution of the mollusca thereby influenced. Thus the Medi- 

 terranean Sea, from a depth of 182 metres (100 fathoms) to its 

 bottom, 2743 metres, maintains almost constantly the tempera- 

 ture 55° F., or through a thickness of 2500 metres. There are 

 known to be ocean currents of both warmer and colder water 

 having definite direction and continuous flow, and by means ot 

 i^ 



* A French metre = 394- inches, therefore 3 metres rather more than 

 equal a fathom ,6 feet^. To reduce the Centigade readings to those of 

 Fahrenheit's thermometer, CO|_[-32 = FO. 



