INTRODUCTION. 



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. To 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, differing 

 specifically from those of the Great Oolite, in 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 in 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 in the vicinity 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 



