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sembling the minute stagnant pool muscle or bivalve, if 

 not one of the same species as now exists, cover it in every 

 division very abundantly ; and like other fissile Stones, the 

 bivalves show the convex on one side and the concave on 

 the other. I am obliged to my friends Messrs Borrer and 

 Weeks for the specimens in my cabinet. The next specimen 

 is from Stonesfield, a quarry famous for a great variety of 

 animal exuvice, especially of the genus Cornu ammorris. 

 This has an alligator's tooth finely preserved. If we may 

 be allowed to judge from the teeth, there have been many 

 species of alligators enveloped in the catastrophe that made 

 such wonderful havock. The other two pieces, the upper 

 of which is without any appearance of petrifaction, are a 

 sort which is often divided by very silvery-looking Mica 

 covering the fissile surfaces, and mostly accompanying the 

 more sandy sort : — we find by Lady Wilson that it ap- 

 proaches even to a Jasper. The lower specimen has casts 

 of different bivalves, and has a hole in the top, which shows 

 in a small degree the manner of manufacturing them for 

 Slates. Several of the above specimens contain a consi- 

 derable portion of Sand. All Limestone quarries have a 

 more fissile Stone near the top : those of Portland, Purbeck, 

 and Bath are generally shelly, and sometimes merely a 

 congeries of compressed shells almost as thin as paper, 



