INTRODUCTION. 

x 
THe authors of the present Monograph, after due consideration of the materials at 
their disposal, have thought fit to limit their illustrations to the Testacea of the Great 
or Bath Oolite; a term under which they would include the series of beds situated 
between the Fullers-earth strata upon which they repose, and the Bradford clay to which 
they are subjacent. ‘lo have enlarged the plan, so as to include the Testacea of the Corn- 
brash and Forest marble, would doubtless have been more comprehensive ; but in the 
present state of our knowledge, the advantage would have been rather apparent than 
real. It will be found that the very few univalves which have been assigned to 
those deposits are almost without exception contained likewise in the Great Oolite, and 
will be found in the Monograph. It is, moreover, not impossible, that at some future 
time a series of univalves may be obtained from the Cornbrash, or Forest marble, differmg 
specifically from those of the Great Oolite, m which case a separate Monograph, or an 
appendix to the present one, might be given. 
It is much to be regretted, that collections of shells should have been procured 
from so few situations in the long course of the formation im this country; and when 
it is remembered, that the Great Oolite constitutes a member of that series of secondary 
rocks which first engaged the attention of geologists, some surprise will mingle with 
our regret. The defect, however, would appear to be of easy explanation. ‘The shells 
do not lie upon the surface, or become separated from the matrix by the action of 
the weather; they are to be procured only by carefully working away the investing 
stone when practicable, which is not always the case: there are likewise large areas 
constituting, probably, the greater portion of the formation, which are altogether des- 
titute of organic remains, or contain only a finely comminuted shelly drift; the areas 
containing assemblages of well-preserved shells, would appear to be of small extent, and 
the presence of several of these im the vicmity of the residence of one of the authors, 
together with the great profusion of undescribed testacea which they have produced, 
have constituted the principal inducement to the present attempt of describing them ; 
these favorable circumstances have enabled them to ascertain the position and vertical 
range of the species with a greater degree of accuracy than would otherwise have 
been possible. 
Beyond the limits of the Minchinhampton district, the number of species procured 
1 
