Vol. 57.] OF ST. lYES AND ELSWORTH. 85 



be some divergence of view as to the exact horizon of the Elsworth 

 Rock, the difference was really very slight ; for, palaeontologically, 

 the lower part of the Corallian Series was Oxfordian. Ammonites 

 of the type of cordatus (now called Cardioceras) are common to the 

 St. Ives Clay and the Elsworth E,ock, just as they are common to 

 the Oxford Clay and the Lower Calcareous Grit elsewhere. Both 

 rocks, in fact, are in the zone of C. cor datum. Ammonites of the 

 type ol perarmatus (now called Asjjidoceras) have a somewhat wide 

 range in time ; but the particular species perarmatus characterizes 

 the upper part of the cordaius-zone, and if it be present in the 

 Elsworth Eock it practically fixes the horizon. Ammonites of 

 the type of plicatilis (now called Perisphinctes) have a still wider 

 range ; and unless the actual species plicatilis (or, as it ought really 

 to be called, hijplex) were intended, the presence of the genus merely 

 suggested rather an Oxfordian than a Corallian horizon, since, 

 taken as a whole, there were more species of the genus in Oxfordian 

 than in higher strata. 



Mr. W. Whitaker also spoke. 



The Author — after thanking the Fellows present for the kindly 

 manner in which they had received his paper, and regretting that 

 want of time had prevented him from laying all the available evidence 

 before them, — in reply to Prof. Seeley and Mr. Hudleston, dwelt 

 upon the great abundance of ammonites of the plicatilis-iy^Q in the 

 Elsworth and St. Ives Rock, and on the calcareous bands of the 

 Ampthill Clay, as distinguishing these beds from the Oxford Clay. 

 He pointed also to the recorded occurrence of Cidaris jiorigemma 

 in the St. Ives Rock. He believed that there was a lithological 

 difference between the Ampthill and Oxford Clays, and would refer 

 to the occurrence of typical Elsworth Rock, in which he had himself 

 found Cidaris-fiorigemma spines, intimately associated with the 

 Corallian limestones at UpAvare, in support of the Corallian age of 

 the Elsworth Rock. In reply to Prof. Blake, he said that he had 

 iJways believed-ammonites of the plicatilis-ty^Q to be characteristic 

 of the upper part of the CoraUian. 



