34 



TYPE AMMONITES— VI 



Oct. 



all of which have rather similar bifurcating ribbing, with periodic swollen 

 ribs parted by deep constrictions, may be given as follows : — 

 Lydistratites — primary ribs long, lobes long 

 Pallasiceras — primary ribs long, lobes short 

 Paravirgatites — primary ribs short, lobes short 



A summEiry of the Isle of Purbeck sequence may now be given- 

 It seems to disclose a quite unexpected result, namely, that the position 

 of a part of the Volgian Stage of Russia is in the Portland-Sands series 

 of Dorset and is lacking from any sequence of the Buckinghamshire 

 beds. Therefore it will be necessary to interpolate a Virgatitan Age 

 in the chronological scale. 



F. Summary — Isle of Purbeck 



Strata 



Fauna 



Age & Hemera 



Portland Stone : 



s. Building Stone, etc. 



r. Flinty Series 



q . SheU Bed 

 Portland Sands : 



Cement Stone Series 



p. Parallel Beds 



0. Marl 



n. White Cement Stone 



m. Marl 



1. Blue Cement Stone 

 k. Marl 



Portland Sands : Sandy Series : 

 h. Massive Bed 



g. Sandy Marl 

 f. Thin Sandstone Bed 

 6. Blue Marls 

 Kimmeridge Clay : 

 d. Dark Clays 

 c. Rhynchonella Marls 

 b. Lingula Shales 

 a'. Rotundum nodules 



a. Crushed Ammonoid 

 Shales 



Kerberites 



Leucopetrites 



(PI. DCLXXVII) 



Biplicate Amm. 



Rhynch. portlandica, 

 Virgatites 



R. subvariabilis ? 

 Lingula " ovalis ' 

 Lydistratites, 

 Pallasiceras 



Paravirgatites ? 



Gigan titan 

 Behemothan 

 kerberus 



leucus 



aquator ? 



Virgatitan 

 scythicus 



Para virgatitan 

 lyditicus 



paravirgatus ? 



A provisional correlation and dating of the strata of Long Crendon, 

 Bucks (compare T.A. IV, 26, V, 71), of Swindon, Wilts (T.A. IV, 28), 

 and of the Isle of Purbeck may now be presented. Opportunity has 

 been taken to give the thicknesses of the Long-Crendon beds ; not that 

 thickness is of any importance in chronological work, especially when, 

 as in Bucks-Oxon, there is rapid variation in a few miles. 



The names printed in capitals of the beds of the Isle of Purbeck 

 were given to me by the manager of the Worth Stone Quarries ; others 

 are from Blake and Woodward. Those below the Shell Bed are a rough 

 synopsis of my researches in company with Mr. Waddington. 



