Yol. 68.] ON MOLLTJSCA EBOM THE ARCTIC BED. 239 



but they are apparently true corneum and not Sj)7i. mcenanum 

 Kobelt, while they are certainly not S2^7i. solidum jSTorm. The 

 species ranges in Europe to the isotherm of 8*^ to 10° C. and in 

 Siberia to that of 10° to 12° C. 



PisiniuM suriNTJM A. Schmidt. Young specimen. (PL XVII, 

 figs. 9 « & 9 b.) 



PisiDiuM OBTUSALE (Lamk., ? Pfr.) Jenyns. 



PisiDiuM NiTiDTJM Jcuyns. (PI. XVII, figs. 10 a & 10 6.) 



and 

 PisiDiTJM PusiLLTJM (Gmcl.) Jcnyns. (PI. XVII, figs. 11 a & 11 h.) 



These species are represented by a few stunted valves, obviously 

 denoting unfavourable conditions. There has been so much con- 

 fusion among the Pisiclia that it is difiicult, if not impossible, to 

 state anything as to their range ; but it would appear that these 

 three species range in Europe as far north as the isotherm of 

 8° to 10° C. 



Conclusion. 



The whole assemblage indicates the fauna of a marsh or swamp. 

 There is practically no other British deposit that is actually 

 analogous. The Pleistocene beds at Swalecliff, near Heme Pay, 

 and the Brickearths, near Chelmsford, are probably of the same 

 age ; but in both these instances the number of species is very 

 small as compared with that of Ponder's End. 



After careful consideration of the matter, we venture to think 

 that the deposit denotes a July isotherm of 8° to 10° C. — that is to 

 say, the temperature of Southern Iceland or Lapland. 



The mollusca from Ponder's End can be divided into two groups : 

 one containing Sphyradhim columella, Vertigo parcedeniata, 

 Planorhis arcticus, and Jaminia muscorum var. lundstroemi, which 

 are all boreal species ; while those of the other all have a wide 

 distribution. Practically, all of this latter group are dwarfed, 

 and clearly indicate the existence of unfavourable conditions, which 

 may well be supposed to have been due to the prevalence of a Ioav 

 temperature. 



It is noteworthy, too, that a number of species which we might 

 reasonably expect to be present are absent, such as Planorhis corneus 

 (Linn.), P. carinatiis (Miill.), P. umbilicaius (MiilL), P. vortex 

 (Linn.), Pliysa fontinalis (Linn.), Ancylus Jfuviaiilis Miill., 

 Bitliynia leacliii (Shepp.), B. tentaculata (Linn.), and Valvata 

 crisfata Miill. These all range as far north as the isotherms 

 of 13° C. or 14° C, and it is not unreasonable to assume that 

 their absence may be accounted for by the fact that the July 

 temperature was probably below 13° C. Taking into consideration 

 the range of the boreal species, a July isotherm of 8° to 10° C. 

 seems most likely to be the correct one for the period. 



At the eastern end of the section the overlying Holocene beds 

 can be seen, although the line of demarcation between the two 



