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 Spirule, 
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 oceur- 
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 which 
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 Pholadomyze, Homomyz, Cercomyze, Myopsides, Gresslyz or 
Pleuromye, the Arcomye and Ceromyz, 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 
