Vol. 68.] OiN^ MOLLTJSCA PKOM THE ARCTIC BED. 237 



This is a Holarctic species, the limit of which in Iceland is the 

 isotherm of 8° C. ; in Norway, Sweden, and Finland 8° to 10° C. ; 

 in Siberia 10° to 12° C. ; while in the Alps its vertical limit is 5° C. 

 Judging from the description, these shells belong to the variety 

 lapponica West, now living in Lapland. 



Identical shells are in our collection from the Pleistocene of 

 Chelmsford (Essex), Swalecliff (Kent), and Stuttgart (Germany) ; 

 while examples from the Pleistocene of Barnwell (Cambridgeshire) 

 and Crayford (Kent) come near to them. 



LiMisr^A sTAGNALis (Liuu.). 



This is only represented by an apical fragment. It is a Holarctic 

 species, the northern limit of which in Norway, Sweden, and Fin- 

 land is the isotherm of 8° to 10° C, and in Siberia 10° to 12° C. ; 

 while its vertical Alpine limit is 13° C. 



Planorbis crista (Linn.). 



A single dwarfed example only of this species was found. 



This is a European form, the northern limit of which in Europe 

 is the isotherm of 12° C. ; while its vertical Alpine limit is that of 

 14° C. 



Planoebis arcticus Beck. (PI. XVII, figs. 8 a & 8 6.) 



Bather rare, only six examples being detected. 



It is a Holarctic species with a decidedly northern range. In 

 Iceland and Greenland its limit is that of the isotherm of 4° to 5° C, 

 and in Norway 12° C. The examples are quite indistinguishable 

 from specimens in our collection which were brought from West 

 Greenland. P. arcticiis is not uncommon in the Pleistocene of 

 Crayford (Kent), and it also occurs in lacustrine deposits near 

 Edinburgh, the age of which is still a matter of doubt. 



Planorbis spirorbis (Linn.). 



Bather rare, about eight examples having been found. 



This European species has a range in Iceland to the isotherm of 

 10° C. ; in Norway, Sweden, and Finland to 14° C. ; and in Siberia 

 to 12° to 14° C. ; while in the Alps it does not occur above the 

 isotherm of 10° to 11° C. 



All the examples from Ponder's End are decidedly dwarfed, and 

 resemble those from the Pleistocene of Chelmsford (Essex). 



Planorbis letjcostoma Millet. 



Bare, being represented only by three dwarfed examples. 



This is a European species, the northern limit of which is the 

 isotherm of 12° C. It is often Considered as only a variety of 

 Sjpirorhis Linn., yet it is noteworthy that both species occur together 

 in this deposit without any intermediate forms being seen. 



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