169 



fact that boulders are always found nearly south of the moun- 

 tain ranges from which they have been originally transported), and 

 that many of the shells are found fossil in the Sicilian tertiaries, 

 this wide distribution may be fully accounted for. As these frigid 

 conditions were gradually altering to more genial ones, those species 

 requiring a lower temperature would gradually die out, and only con- 

 tinue to exist in higher latitudes. The littoral and shallow water 

 species would be most affected by such an alteration of climate ; 

 and while the fauna of the littoral and laminarian zones would be 

 entirely changed, — the shells composing that fauna replaced by other 

 species, — those living in the deep sea would continue to exist, per- 

 haps at a greater depth, mingled with the species brought in with 

 the new physical conditions of the area. This we know to be the 

 case ; for while the northern littoral shells, such as Mya truncata, 

 &c, are found only fossil in Sicily, many of the deep-sea arctic species 

 that existed there when those fossils were alive are still found living 

 in the deeper parts of the Mediterranean. 



The same thing occurs on our coasts, where the arctic littoral 

 or shallow-water shells, as Astarte arctica, T'ellina proximo,, Natica 

 helicoides, &c, which are found in shallow water on the Scandi- 

 navian or Greenland coasts, are now rare as deep-sea shells, and that 

 in the same area in which they were formerly abundant as shallow- 

 water species. Some species are capable of enduring great differ- 

 ences of climate, the Mytilus edulis being found as abundantly on 

 the coast of Greenland as on our own shores. 



It is not difficult to account for the presence of the southern species 

 on the coast of Norway, as the Gulf-stream sets directly along the 

 coast, warming its waters, and rendering them habitable for species 

 requiring a moderately high temperature. The great abundance and 

 wide distribution of these species show that the present order of 

 things has continued for a great length of time. The gradual ex- 

 tinction of northern shells on our coasts is still going on ; the 

 number of living specimens of Pecten danicus is very small, while 

 dead shells are very abundant, and fresh dead specimens of Pecten 

 islandicus are frequently dredged, though a living specimen has not 

 yet occurred. It is probable that this species has died out very 

 recently. 



On the eastern shores of Davis's Straits the Mollusca are about 

 half as numerous as on the coast of Norway. The fauna differs in 

 the prevalence of arctic types and the total absence of southern. At 

 a former period the fauna was of a mixed character ; species now in- 

 habiting more southern latitudes are found fossil in the raised beaches 

 at Disco Island, which species are no longer found living on the coasts. 



April 28, 1856. 



A paper was read " On the Theory of Heat," by Mr. A. A. Har- 

 rison, of Trinity College. 



The object of this paper was to show that there is considerable 



