Air, 86° 



N. lat. 7° 30' 



„ 85-5 



„ 5 24' 



„ 67 



„ 24 38' 



!; 



S. lat. 34 



2 ZOOLOGY OF THE VOYAGE OF THE SULPHUR. 



and I shall briefly show, that in any given latitude, the circumstances under which 

 animals live are very different in these localities. Within the tropics the shores 

 are Mashed by water of a high temperature and of much uniformity, usually 

 ranging from 80° to 87°, and during a portion of the tide many are left exposed 

 to the unobstructed rays of the sun on rocks of a very great degree of heat, such 

 as the species of Littorina, Patella, Siphonaria, and JBalanus. Whilst, in the near 

 neighbourhood, at a few fathoms beneath the surface, the temperature will be 

 still more uuiform, but less by fifteen or twenty degrees. Some observations 

 were made at different depths with the self-registering thermometer to ascertain 

 these circumstances. 



Quibo, Veragua 30 Fathoms, temp. 55°60° Surface, 82° 



China Sea 84 „ „ 66 „ 84 



Bay of Magdalena 6 „ „ 78 „ 69 



Cape of Good Hope 65 „ „ 62 „ 64 



If temperature alone influenced molluscous animals in the selection of 

 locality, and knowing as we do that within the tropics the high temperature of 

 the surface water gradually declines till at 100 fathoms it is usually about 49°, 

 and at 1000, 44° ; then we might expect that as we descended we should meet 

 with species corresponding to those which inhabit the shores of progressively 

 higher latitudes. Our operations were not usually conducted in very deep water, 

 or often above fifty fathoms; for we found by experience that deep dredgings 

 endangered the safety of our machinery, and we had rarely during the voyage 

 the means of replacing it. But under this depth I cannot say that I have 

 observed this to be the case, and we are then compelled to admit the influence of 

 incumbent pressure, and perhaps other causes. There are several groups which 

 in a very marked manner select situations in deep water for their abode, as 

 Cancelluria, nearly all the Pleurotoma, Marginella, Nucula, Corbula, and Tere- 

 bratula. The numerous species of Nucula are found from the Polar seas of both 

 hemispheres to the Equator, but are always abysso-pelagic. There are others 

 which are found in both situations, and particularly the full genus Chiton. It 

 will, however, be seen that the species are here very careful in the selection of 

 their locality ; those which are littoral will not be found in deep water, and vice 

 versa ; some prefer dark situations, under stones, or in the fissures of rocks above 

 low water; others precisely at the limits of low water, or slightly submerged; 

 others, again, at the depth of a few fathoms ; and, lastly, others at considerable 

 depths. At Panama a species of Pecteu was found at a depth which subjected it 

 to a pressure equal to upwards often atmospheres. These are the circumstances 

 to which, though exceptions may be found, they are as a rule strikingly obedient. 



The peculiarities which molluscous animals display under increased pressure, 



