DK JOHNSTON S CATALOGTJE OF BIVALVED SHELLS. 77 



mation of these curious substances ; and this being admitted, the 

 various peculiarities of figure observable in them may easily be ac- 

 counted for. Those, for instance, with a protuberance to one side 

 more than another, would be deposited on the surface 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 ex- 

 plain any variety of figure in those specimens which have a flattened 

 base, and which thus shew that they must at one time have adhered 

 to a level surface. Those again which have an oval shape, composed 

 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 the imbedded 

 boidders, — 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 

 vaUey, 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 shaU not pretend to determine. 



A Catalogue of the Bivalved Shells found on the Coast of Berivichhire and 

 North Durhatn. By Dr Johnston. 



SuBKiNGDOM MOLLUSCA. Class CONCHIFEEA. 



I. MONOMYAIEIA. 



OsTREADa;. Anomia punctata. 



Anomia ephippium. aculeata. 



squamula. Ostrea edulis. 



