116 
Mr. J. Lycett on the Fossil Conchology 
were in process of deposition, a molluscous class (the carnivorous 
Trachelipods), which in our present seas perform the office of 
keeping down within due limits the other molluscous races, did 
not then exist, or that they were extremely few, and that it was 
only on the extirpation of those extensive genera of Cephalopods, 
the Ammonites and Belemnites, at the commencement of the 
tertiary epoch, that the carnivorous Trachelipods made their ap- 
pearance. Living in a district distinguished by a great profu- 
sion of molluscous remains, a large proportion of which are abso- 
lutely unknown to science, a favourable opportunity for testing 
the correctness of the foregoing theory was presented to me, 
more especially as these remains occur in an unusually good 
state of preservation, extending in some instances even to the 
original colours of the univalves, the hinges of the bivalves, and 
the external ligament of the hinge in the latter shells. Before 
however stating the results of this inquiry, a very brief sketch of 
the physical and geological characters of the district may not be 
unacceptable to the members. 
A circle having a radius of only four miles, with the town of 
Minchinhampton in the centre, will comprise the whole district 
to which these fossils refer. The Bath Oolite, or Compound 
Great Oolite as it is now termed by geologists, is the uppermost 
formation ; its continuity is however broken by two great valleys 
of denudation, the vales of Brimscomb and Woodchester, which, 
with their numerous lateral ramifications, have cut through the 
whole series of rocks from the upper part of the Great Oolite to 
the middle of the lias inclusive, having a mean depth of 500 
feet, thereby producing a combination of circumstances eminently 
favourable for exposing the useful beds of stone and conveying 
it by water-carriage. 
The divisions of the Compound Great Oolite are. Great Oolite 
and Bullcr^s Earth, the former having a thickness of 130 and 
the latter of 70 feet. At some few localities the base of the Great 
Oolite has one or two beds of true Stonesfield slate associated 
with brown marls. In this respect however, as in the mineral 
character of the formation generally, the greatest variety and 
uncertainty exist ; opposite sides of the same quarry will often 
exhibit such a change ; thus an oolitic and shelly limestone will 
pass into a barren sandstone. Keeping this fact in view, a con- 
siderable latitude must be allowed in the following arrangement, 
which is given only as a general and approximate view of the 
whole series of beds. The Great Oolite proper may be con- 
veniently subdivided into three scries of beds, an upper and 
lower fossiliferous, often serviceable for building purposes, and 
a middle, more barren and unserviceable. Beginning with the 
uppermost, or those which immediately underlie the Bradford 
