MOLLUSCA FROM THE GREAT OOLITE. 3 



examples of this class of carnivorous mollusks are here few, both as to number of species 

 and of individuals. This fact, together with the circumstance that they do not mark any 

 particular stratum, renders it highly probable that they were not associated, when living, 

 with the denizens of these shelly beds, but, like dead shells of the recent Spirulae, 

 individuals occasionally floated upon the surface, and were wafted to some coast or shelly 

 strand, often very distant from their real habitat. With the chambered shells such occur- 

 rences may have been common ; the air-tight little vessel, separated by decomposition from 

 the animal, would ride upon the wave, and only suffer injury upon striking the ground of 

 the beach. A consideration of the gregareous habits of the several families of recent, and 

 probably also of extinct Cephalopoda, would lead us to regard an occasional stray individual 

 as having travelled from some colony more or less distant ; but the beds of closely-packed 

 Ammonites, of every stage of growth, which occur in certain of the Jurassic rocks, would 

 appear to be the effect of occasional rapid earthy deposits, which took place during that 

 seasonal period when the Mollusks, lying torpid and contracted within their shells, were 

 at once entombed in that condition. We have also an explanation of the perfect condition 

 which the Ammonites of these beds usually exhibit ; the place of retirement would be 

 exempt from the turbulence of a shallow sea, and exposed only to the deposit of mud or 

 other fine sediment, which would protect the shells from injury. In the few Ammonites 

 and Nautili of the weatherstone beds, we see the reverse of these conditions ; those large 

 and fragile shells, exposed in that detrital deposit to every kind of attrition and accident, 

 are very rarely perfect ; seldom more than two continuous chambers can be found Avhich 

 have not been invaded by earthy sediment, and often large portions of shell are wanting 

 altogether. The paucity of the Brachiopoda in these beds is also worthy of notice. Three 

 species of Terebratula are found associated with nearly 400 species of Mollusks; and 

 certain genera, which are peculiarly prominent in the Oolitic rocks generally, are mostly 

 absent ; of these genera, the Pholadomyaj, Homomyse, Cercomyae, Myopsides, Gresslyse or 

 Pleuromyse, the Arcomyee and Ceromya3, being exceedingly rare. The greater number of 

 these genera are not uncommon in the limestones or upper beds of the Great Oolite, and 

 occasionally, also, in the lower beds or sandstones, when they are separate from any shelly 

 deposit. 



The section of the shelly beds, exhibited by the great quarry upon Minchinhampton 

 Common, affords a clear view of their distinctive characters and order of superposition. 

 The upper part consists of thinly-laminated stone, five or six feet in thickness ; to this 

 succeeds the beds usually termed planking, a designation implying a thin bedded stone, 

 out occasionally consisting of beds of great thickness : fourteen feet would appear to be 

 their utmost thickness. They mark the downward limit of our new genus Purpuroidea, 

 in the lowest bed of which it is very abundant. 



An uncertain and variable stratum, of a few inches, of sandy marl next succeeds, in 

 which the few casts of bivalve shells hitherto found have the valves in apposition. To this 

 succeeds thin-bedded yellowish sandstones, nearly destitute of shells, and worthless for 



