266 



ME. T. EOBEETS ON THE UP PEE JTJEASSIC EOCES 



Ten species occur in the Purbeck : — 



Cypris purbeckensis, Forbes. 

 Pbysa Bristowi. Forbes. 



■ wealdiensis, Maillard. 



Limnasa physoides, Forbes. 

 Leptoxis subangulata, Sandb. 

 Valvata helicoides, Forbes. 



Hydrobia Chopardi, Sandb. 

 Corbula duristonensis, Maillard. 



Forbesii, De Lor. 



Protocardia purbeckensis, Mail- 

 lard. 



He further adds that the fauna of the upper brackish beds is 

 found exclusively in the upper part of the Middle Purbecks of 

 England. Maillard (op. cit. p. 134) gives the following table of 

 correlations : — 



Jura. Angleterre. 



Couches Saumatres superieures. 

 Couches nympbeennes. 



Middle Purbeck. 



Niveaux a fossiles Saumatres. 



Faunes principalement d'eau 

 douce. 



Marnes a gypse. 



Dirt Beds. 



Lower Purbeck : Gypse a Durlstone 

 Bay. 



Dolomie saccharoide a Corbula in- 

 Jlexa et Cyrena rugosa. 



Portlandien a Cyrena rugosa. 



The palseontological evidence is undoubtedly in favour of the cor- 

 relation above mentioned. If so, then the representatives of oar 

 Upper Purbecks must be sought for in the Yalangian of the Jura ; 

 the latter, however, is a marine deposit, and the former freshwater, 

 so that in the absence of fossil evidence the correlation must be 

 made purely on stratigraphical grounds. 



In the sub-Wealden boring it was shown that the lower portion 

 of the Purbeck Beds contained a quantity of gypsum, which appears 

 to be somewhat similar in character to that of the lower part of the 

 Purbeckian of the Jura. If our correlation be correct, it is rather a 

 remarkable coincidence, that the same physical conditions prevailed 

 during a portion of this period at such widely separated localities. 



In the subjoined table an attempt is made to summarize the 

 correlations arrived at in this part of the paper. It would appear, 

 in some cases at least, that the changes in the physical conditions 

 which caused the termination of one stage and the commencement 

 of the succeeding one did not take place synchronously in England 

 and in the Jura, and on this account the faunas, as it were, some- 

 times overlap. Hence the difficulty of exactly defining the equi- 



