DR JOHNSTON'S CATALOGUE OF BIVALVBD SHELLS. 77 



and this being admitted, the various peculiarities of figure observable in 

 them may easily be accounted for. Those, for instance, with a protu- 

 berance to one side more than another, would be deposited on the sur- 

 face of the boulder inclined at a considerable angle ; those again with 

 the most perfect shapes, would be formed on the under side of the stone 

 in a vertical position, like a regular stalactite ; and so we might equally 

 easily explain any variety of figure in those specimens which have a 

 flattened base, and which thus shew that they must at one time have ad- 

 hered to a level surface. Those again which have an oval shape, com- 

 posed as it were of two distinct hemispheres, we may suppose to have 

 been formed in this manner, viz. by the water flowing over a hollow on 

 the surface of the mass or boulder, that this hollow will gradually be 

 filled by the particles of the fine clay, which, as they will naturally have 

 more attraction for themselves than the smooth surface of the rock or 

 stone they are deposited on, will thus go on increasing, layer after layer 

 being added, until at length the figure of an oval or double hemisphere 

 is assumed. 



It is easy likewise to explain how they occur so often single, and are 

 generally found in the bed of the streamlet. A very violent shower of 

 rain would account satisfactorily for both appearances. The violence of 

 the water flowing over the banks would loosen many of these imbedded 

 boulders, they would rush furiously down the slope, from the rough- 

 ness and resistance of their course they would part with their attached 

 fairy stones : these would thus be carried into the bed of the stream, where, 

 as before remarked, they are generally found, and that too in greatest 

 abundance after violent showers of rain. 



It has been remarked by the inhabitants of the neighbourhood, that 

 they are much less abundant now than they once were, at least good 

 specimens ; but whether this be owing to their having been more sought 

 for of late years by the increased number of visitors to this valley, or 

 whether it is owing to the higher grounds above the Dean being more 

 drained than formerly, and consequently allowing less water to trickle 

 down their slope, I shall not pretend to determine. 



A Catalogue of the Bivalved Shells found on the Coast of Berwickshire 

 and North Durham. By Dr JOHNSTON. 



SOBKINODOM MOLLUSCA. CLASS CONCHIFERA. 

 I. MONOMYAIRIA. 



OSTREADA. Anomia punctata. 



Anouiia ephippium. aculeata. 



squamula. Ostrea cdulis. 



