Oolitic Formations in the vicinity of Minchinhamjdon. 251 



when young and the size of a nut, has tubercles in Ueu of spines ; 

 the transverse ribs are well-defined ; but the longitudinal eleva- 

 tijons which give the species a rugose aspect are absent, the basal 

 notch is nearly obsolete, and the columella is nearly straight. 

 The P. nodulata has still greater varieties ; when full-grown it has 

 two encircling rows of tubercles on the body-whorl, from the 

 lower and smaller of which proceed oblique longitudinal ribs 

 which terminate in a transverse elevated 

 basal belt. The young shell is nearly 

 smooth ; the smaller circle of tubercles is 

 scarcely distinguishable ; the ribs are ab- 

 sent, as is likewise the basal belt. Oc- 

 casionally in full-grown specimens the 

 smaller circle of tubercles degenerates 

 into an encircling rib. The spire is of 

 various degrees of elevation, in fact 

 scarcely any two specimens are exactly 

 alike ; a considerable number are there- 

 fore desirable for its full elucidation. The p. nodulata, yovLU^. 

 third species, P. glahrata, is rare ; it equals the others in mag- 

 nitude. Tt is seldom that we can trace the limits of a spe- 

 cies over any particular area ; here however we are enabled to 

 do so with tolerable accuracy. These shells are grouped together 

 in the blocks of stone by hundreds, occupying a vertical thick- 

 ness of 5 01' 6 feet, and spread over an area 50 yards wide and 

 100 long. It is to be regretted that this prolific space will ere 

 long be entirely removed, and the Purpuroidea in its perfect state 

 will probably be only a matter of tradition as far as this vicinity 

 is concerned. These conditions have produced upon our mind 

 the impression that here we perhaps behold the birth-place or 

 original seat from whence the difi'usion of the genus took place. 

 Repeated observations have 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 the 

 rock can only be separated in the form of the Natica-\\kQ casts. 

 The Patellce, 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 pecu- 

 liarities. 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 much of the rugose aspect caused by the 

 lines of growth, the longitudinal stride are faintly marked, and 

 the shell is altogether extremely thin. In a very young state 



