near St. Andrews. 203 



plants, with some of the stems in an upright position. There 

 were likewise numerous fibres and rootlets of seemingly various 

 plants. These rootlets and impressions clearly indicate that 

 marshy and perhaps other plants had at one time grown in 

 abundance on this bed ; and, so far as observed, this would ap- 

 pear to have been before the third bed was laid down, as no 

 rootlets or stems could be seen in that deposit ; and there seems 

 no reason why they should not have been preserved, or traces of 

 them, in the one bed as well as in the other, if they had ever 

 been there. No doubt, from the littoral character of the shells 

 found in this bed, some of them could exist although they were 

 not many feet under water at every tide. Some of the Tellina 

 can live high upon the muddy shores of estuaries; so can Scro- 

 bicularia. And, from the large size of these species, it may 

 be inferred that they had found a more congenial habitat than 

 seems to have been the case with Mytilus and Cardium, whose 

 size and thickness might at the same time be somewhat influ- 

 enced by the freshness of the water. The three latter species 

 can undoubtedly live in places where the water is as often fresh 

 as salt. Jtffnys says* that the Mussel and the common Peri- 

 winkle [Littorina littorea) are occasionally found living on the 

 shore in a stream of perfectly fresh water during the recess of 

 the tide, that Cardium edule has the same habit, and that the 

 latter species even occurs associated with freshwater Mollusca. 

 It is well known, from the writings of Montagu and others, 

 that Scrobiailaria piperata burrows in muddy places that are 

 occasionally covered with fresh water, at the mouths of rivers, 

 or far up estuaries. So far as ray own obsenations go, the 

 shell of the latter species retains nearly its normal size and 

 thickness in places where neither Cardium nor Mytilus seem 

 capable of existing, or, if met with, are always in a thin and 

 dwarfed state. However, there can be no doubt that all the 

 foregoing species could not have lived in situations so high 

 above the sea as to allow even marshy plants to grow. This 

 being the case, the plants must have grown after the bed was 

 raised a few feet higher above the water. For this purpose 

 one of two causes would be necessary : either the land at this 

 time was slowly rising, or the estuary was gradually silted up : 

 perhaps both these operations were going on at the same time, 

 although it would be impossible in the present instance to say 

 which had the chief hand in the matter. At the same time that the 

 land was elevated to some extent, there seems no room to doubt 

 (indeed, according to the observations of Geikief and others, there 

 ^pear good reasons for believing) that the land on many other 



* British Concholo^-, vol. ii. 

 t The Glacial Drift "of Scotland. 



