CHAP. IX.] THE DEEP-SEA FAUNA. 461 



Arctic Expedition, which reached 2,600 fathoms, 

 when a Cuma and a fragment of an Astarte came up 

 in the ' Bulldog ' machine. He adds, " It is evident 

 that the majority, if not the whole of our submarine 

 (as contradistinguished from littoral or phytopha- 

 gous), moUusca originated in the Kortli, whence they 

 have in the course of time been transported south- 

 wards by the great Arctic currents. Many of them 

 appear to have found their way into the Mediterra- 

 nean, or to have left their remains in the tertiary or 

 quaternary formations of the south of Italy ; some 

 have even migrated into the Gulf of Mexico." 



I have great hesitation in questioning any of the 

 conclusions of my friend Mr. Gwyn Jeffreys on a 

 subject in which he is so excellent an authority, 

 Ijut I confess I do not quite see the cogency of his 

 reasoning on this point. It would seem rather that 

 the last change in the moUuscau fauna of the British 

 area, at moderate depths, consisted in the retirement 

 of northern species at the close of the glacial period 

 and the immigration of southern forms. The qua- 

 ternary beds of the Clyde district contain a rich 

 assemblage of mollusca ; those of the neighbourhood 

 of Eothesay especially representing the deeper part 

 of the Laminarian and the Coralline zone. The 

 broad characteristic of the fauna of this bed is 

 that many of the most numerous species — for 

 example, Pecten islandicus, Tellina calcarea, and 

 Natica clausa — are now extinct in the seas of 

 Britain, but are still met with in abundance in the 

 seas of Scandinavia and Labrador ; while many forms 

 now extremely common in the British seas and 

 having a southern extension are entirely absent. 



