PRESTWICH BRITISH AND FOREIGN TERTIARIES. 103 



It would appear that this division of the Calcaire grossier is most 

 largely developed towards the centre of the Paris basin. As it trends 

 westward into the department of the Oise, the lower beds become more 

 mixed with green sands, and the calcareous strata become thinner. 

 In England the calcareous beds, properly so called, are entirely 

 wanting, being replaced by yellow and green sands, with some cal- 

 careous matter and brownish laminated clays. The abundance of 

 many of the common French species in the middle of the Brack- 

 lesham series leaves, however, little doubt of the contemporaneity of 

 that particular portion with the Grignon and Parnes zone. Never- 

 theless from the circumstance of the green- sand sediment, originally 

 common to the two areas, having continued to prevail, but in a more 

 muddy condition, in the English area, whilst calcareous sands formed 

 at this period the sea-bed in the French area, those creatures which 

 the former conditions more particularly suited, — such as the Num- 

 mulites, and many of the siphonate lamellibranchiates, — continued 

 to live and abound in England after they ceased to be numerous in 

 France. At the same time the excessive development of other Mol- 

 lusca, of which the calcareous sea-bed favoured the existence in 

 France, has no parallel in this country : comparatively few (apparently 

 not 25 per cent.) passed at this period from the one to the other 

 geographical area. 



3. The last division passes very gradually into the next, or third, 

 division. The distinction is more marked in the Aisne than in the 

 Oise, In the eastward of the Paris basin this division is from 4 to 8 

 feet thick ; it soon expands to 10 feet ; and, as it approaches the Oise, 

 attains its maximum thickness of from 25 to 30 feet. In the ad- 

 jacent parts of the Oise it is from 20 to 25 feet, decreasing to a few 

 feet in thickness as it trends further westward. 



A great change here takes place in the character of the organic 

 remains. Instead of the rich marine fauna of the underlying beds, 

 we perceive the setting-in of freshwater conditions, and find a 

 comparatively limited fauna. The marine shells become few and 

 scarce ; brackish-water and sestuarine shells preponderate ; several 

 species of Cerithium are so abundant, that these beds are sometimes 

 called the '' Calcaire a Cerites." Miliolites likewise occur in great 

 profusion ("Calcaire a Miliolites"). Freshwater shells — Paludma, 

 Limncea, and Planorbis — become common in places. Some land- 

 shells are also met with ; the Cyclostoma mumia here occurs for the 

 first time. The number of species enumerated by M. Graves from 

 Mouy, Bonqueval, and Chambors amount to about forty. Of the 

 characteristic species of this division in the Aisne and Oise, only a 

 few occur at Bracklesham ; the following are the principal : — 



Cerithium calcitrapoides. Crassatella rostrata. 



■ cristatum. Cytberea elcgans. 



echidnoides. striatula. 



semicoronatum. Venericardia elegans. 



Tm*ritella fasciata. 



There are others besides which are common throughout. 



Unlike the two preceding divisions, the fossils of this one are suf- 



