28 Mr. T. G. Ponton on Pyrula (Fulgur) carica 



bed inseparable by any great distinction from, and under, sands 

 full of the Potton-Sand fossils*. 



A discussion of the whole question and desci'iptions of the 

 fossils are given in my ' Geology of the Country round Cam- 

 bridge.' It may be here stated that this investigation led to 

 proposing the following classification of the secondary strata : — 



r Chalk. 

 Cretaceous ... s Greensand. 

 L Gault. 



iShankliu (or Lower Green) Sand. 

 Wealden / Pottou ? and Wicken and 

 Purbeck 1. Farringdou beds ? 

 Portland. 



P 1 1th' ( r ^™™6i'i<^g6 Clay. 

 FelspatWc).';!'|g^'f^^^f^^,^^^ Gamlingay Clay. 



r\ YY J Great Oolite. 



1. Inferior Oolite. 



Lias. 

 Trias. 



While these divisions mark approximately the greater physical 

 breaks and the periods when great changes were made in phy- 

 sical geography, it happens almost as a necessary consequence 

 that there is a linking of the life between each of the six great 

 groups of formations here indicated. 



V. — Remarks on Pyrula (Fulgur) carica {Lamarck) and Pyrula 

 (Fulgur) perversa (Lamarck). By T. Graham Ponton. 



Although fully alive to the responsibility which rests upon any 

 one who presumes to doubt the specific value of old and well- 

 known forms, I nevertheless venture to submit the few following 

 remarks to the consideration of other conchologists. 



Having for some time past been engaged in re-arranging the 

 collection of shells in the museum of this city, and having paid 

 particular attention to the species comprised in the Lamarckian 

 genus Pyrula, I have reluctantly come to the conclusion, for 

 reasons to be afterwards mentioned, that Pyrula pej-versa (La- 



* At the meeting of the Cambridge Philosophical Society, May 27, 186/, 

 a paper was read " On the association of Potton-Sand fossils with those of 

 the Farringdon Gravels in a phosphatic deposit at Upware on the Cam ; 

 with an accouut of the superjiosition of the beds, and the significance of 

 the affinities of the fossils." This series I propose to name the Wicken 

 and Herrimere group. I have already obtained 120 species, including 

 many continental species not previously recorded in Britain. 



