MUNDESLEY FRESH-WATER FORMATION. 



T^O 



Ml". King. The last-named geologist has had the kindness 

 to draw up for nie the annexed diagram of the various beds 

 which he has recently studied in detail.* 



The formations 3, 4, and 5, already described, p. 213, were 

 evidently once continuous, for they may be followed for 

 miles N.VV. and S.E. without a break, and always in the 

 same order. A valley or river-channel was cut through them, 

 probably during the gradual upheaval of the country, and 

 the hollow became afterwards the receptacle of the comj)ara- 

 tively modern fresh-water beds, A, B, c, and d. The}' may 

 well represent a silted-up river-channel, which remained for 

 a time in the state of a lake or mere, and in which the black 

 peaty mass, b, accumulated by a very slow growth over the 

 gravel of the river-bed A. In b, we find remains of some of 

 the same plants which were enumerated as common in the 

 ancient lignite in 3', such as the yellow water-lily and pond- 

 wort, together with some fresh-water shells which occur in 

 the same fluvio-marine series 3'. 



Fiff. 3i. 



Paludina marginata Michaud. {P. minuta Strickland.) 

 Hydrohia marginata.f 



The middle figure is of the natural size. 



The only shell which I found not referable to a British spe- 

 cies is the minute paludina, fig. 34, already alluded to, p. 164. 



-■'■ Mr. Prestwioh has given a correct 

 account of this section in a paper read 

 to the British Association, Oxford, 

 1860. See Geologist's Magazine, vol. 

 iv. 1861. 



■j" This shell is said to have a sub- 

 spiral operculum (not a concentric one. 



as in Paludina), and therefore to be 

 referable to the Hydrobia, a sub-genus 

 of Rissoa. But this species is always 

 associated with fresh-water shell*, 

 while the Rissoae frequent marine and 

 brackish waters. 



