DISTRIBUTION OF THE MOLLUSCA 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 
Crag,a few extinct species are found (e.g. Nucula Cobboldie, 
Pl. 17, f. 18). AtChiliesford, 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. TZvrophon scalariforme, Admete viridula, Scalaria groen- 
landica, and Natica grenlandica, also occur in the Norwich Crag ; 
and Astarte borealis, with several arctic forms of T'ellina, are 
amongst the commonest shells, and frequently occur in pairs, 
or with their ligament preserved; the deposit is extensively 
quarried for shell-sand. 
Raised sea-beds with Arctic shells at Uddevalla, in Sweden, 
haye been repeatedly noticed ever since the time of Linnzus. 
Captain Bayfield discovered similar beds near Quebec, 50—200 
feet above the River 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, Cardiwm edule, var. rusticum.t 
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 
Lajonkairti, Chama gryphoides, and species of Cassidaria, Cleo- 
dora, Sigaretus, Terebra, Columbella, and Pyramidelia. It also 
contains a few forms belonging to an earlier age—a Pholadomya, 
a true Pyrula, a Lingula, 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. 
} 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 efficient and probable cause would be a diversion of the Gulf-stream, which 
he supposes to have flowed up what is now the valley of the Mississippi. (Geological 
Journal). 
