DlSTRIBtJTION OF THE MOLLUSC A IN TIME. 131 



species of shells from these " Glacial beds," nearly all of which 

 are now existing in British seas.* 



In most of the localities for glacial shells, the species are all 

 recent ; but at Bridlington, Yorkshire, and in the Norwich 

 Oay, a few extinct species are found (e.g. Nucula Cobholdice, 

 PL 17, f. 18). At Chiliesford, Suffolk, Yoldia arctica and myalis 

 occur of large size and in excellent preservation, with numerous 

 specimens of Mya truncata, erect as they lived, in the muddy 

 sea-bed. Trophon acalariforme, Admete viridula, Scalaria green- 

 landica, and Nativa groenlandica, also occur in the Norwich Crag ; 

 and Astarte horealis, with several arctic forms of TelUna, are 

 amongst the commonest shells, and frequently occur in pairs, 

 or with their ligament preserved; the deposit is extensively 

 quarried for shell- sand. 



Eaised sea-beds with Arctic shells at TJddevalla, in Sweden, 

 have been repeatedly noticed ever since the time of Linnaeus. 

 Captain Bayfield discovered similar beds near Quebec, 50 — 200 

 feet above the Eiver St. Lawrence, containing an assemblage of 

 shells entirely Arctic in character ; whereas in the present gulf 

 he obtained an admixture of the American representatives of 

 Lusitanian types, Mesodesma, Periploma, Petricola, Crepidula. 



The glacial deposits of the northern hemisphere extend about 

 15® south of the line of "northern limit of trees;" but this 

 comparatively recent extension of the Arctic ocean does not 

 appear to have much influenced, if it ever invaded, the inland 

 basin of the Aralo-Caspian, which contains only one species 

 common to the White Sea, Cardium edule, var. rusticum.-\ 



The older pliocene period is represented in England by the 

 Coralline Crag, a deposit containing 340 species of shells. Of 

 these 73 are living British species, but (with two or three ex- 

 ceptions) they are such as range south of Britain. (Forbes.) 

 The remainder are extinct, or living only to the south, especially 

 in the Lusitanian province : e.g. Fossarus sulcatus, Lucinopsis 

 Lajonhairii, Chama gryphoides, and species of Cassidaria, Gleo- 

 dora, 8igaretus, Terehra, Columhella, and JPyramidella. It also 

 contains a few forms belonging to an earlier age — a Fholadomya, 

 a true Fyrula, a Lingida, and a large Voluta, resembling the 

 Magellanic species. 



* The species which have retired farther north are marked (**) in the preceding 

 Arctic List, pp. 57, 58. 



t Mr. Wm. Hopkins, of Cambridge, has investigated the causes which may have 

 produced a temporary extension of the Arctic phenomena in Europe ; and considers 

 the most efBicient and probable cause would be a diversion of the Grulf-stream, which 

 he supposes to have flowed up what is now the valley of the Mississippi. ( Geological 

 Journal) . 



