Oolitic Formations in the vicinity of Minchinhampton. 23 



the Purpuroidea in its perfect 9tate will probably be only a matter of 

 tradition as far as this vicinity is concerned. These conditions hvae produced 

 upon our mind the impression that here we perhaps behold the birth-place 

 or original seat from whence (he diffusion of the genus took place. Repeated 

 observations hare shown that specimens occur in every other quarry in the 

 neighbourhood, but so rarely, that the total number noticed probably has 

 not exceeded twenty in the course of the last six years. Higher in the 

 series they are met with in several beds of compact homogeneous limestone, 

 but much more sparingly than in the planking, and from the hard structure 

 of tire rock can only be separated in the form of the tVatica-Wke casts. 



The Patella, which occur abundantly in our shelly Oolite, like their 

 recent congeners , vary so considerably as sometimes to puzzle even persons 

 who have been accustomed to their peculiarities. The most common species, 

 P. rugosa.) when obtained south of the vale of Brimscomb, fully deserves 

 its name, but north of the vale it loses mueh of the rugose aspect caused by 

 the lines of growth, the longitudinal striae are faintly marked, and the shell 

 is altogether extremely thin. In a very young state the form is a longer 

 oval and much less elevated, the apex being slightly turned to the right 

 side, constituting the Patella ancilloides of the " Mineral Conchology," 

 which species should therefore be expunged. Patella Aubentonensis 

 (Archiac) occasionaly loses its striae altogether, and this change is not 

 confined to any particular size or form of shell. Patella nana in advanced 

 age is spread out more horizontally towards its borders, and forms a concave 

 conical and ovate shell. The fine encircling striae then altogether disappear; 

 the minute figure in the " Mineral Conchology" refers to the shell in its 

 young state. 



The genus Nerinea is represented by upwards of fourteen species, of 

 which five ar<» abundant ; they seem to occupy in the Oolitic rocks the 

 place of the Cerithia of the older tertiary strata, and are decidedly the 

 predominating univalve of their period. Our most common species are 

 destitute of the tubercles or striae by which these shells are usually 

 ornamented ; four only of the species appear to have been figured or 

 described. One fact in connexion with the extinct carnivorous trachelipods 

 should be noticed. The recent genera of that class are furnished with a 

 tubular boring apparatus, by means of which they drill round holes in the 

 bivalves and prey upon their juices. As none of the Oolitic bivalves have 

 such perforations, we may conclude that the extinct carnivorous genera of 

 that period were differently constituted. 



Of the Natica we number fourteen species, seven of which are new ; 

 although the species are thus numerous, one only, N. Michelini (Archiac), 

 is at all common. 



The family of the winged shells, or Strombida> y are represented by 

 upwards of eleven species belonging to the same genus ; the greater number 

 of these likewise occur eveu to the base of the formation. We have 

 separated them from the Rostellarias and Pteroceras under the generic 

 term Rostrotrema ; they are distinguished from the true Rostellarias by the 

 absence of an upper or posterior siphon upon the spire, the outer lip not 

 extending beyond the body whorl or but slightly upon the penultimate, 

 and there is no corresponding thickening upon the inner lip to form a 

 channel. It is true that one or two recent species of Rostellaria have no 

 posterior siphon upon the spire, but in such instances the siphon is present 

 and coiled round upon the upper part of the wing. From Strombus it is 

 sufficiently distinguished by the absence of the sinus on the outer lip. We 

 venture to suggest that the Strombida requires a re-arrangement, the 

 digitations of the outer lip not being of sufficient importance to found upon 

 them generic distinctions : they are of too variable a character, and in 

 some instances depend very much upon the age of the specimen. 



Another generic form, as yet found in no other part of England, is a 

 conical turbinated univalve, called by me Trochotoma ; five species occur, 

 but only one is common. Its distinguishing generic feature is a transverse 



