PRESTWICH BRITISH AND FOREIGN TERTIARIES. 115 



Both deposits are marked by a paucity of Cephalopoda. Not a 

 single species of Nautilus has yet been quoted from either country. 

 Freshwater shells are very scarce in the Barton Clay proper, whereas 

 the Sables moyens, especially the upper beds, contain a considerable 

 number, including several species of Limncea and Cyclostoma, genera 

 not occurring at Barton. In both deposits there is a profusion of 

 molluscs of the genera Fusus, Pleurotoma, Lucina, and Corbula. 



Other evidence therefore of the contemporaneity of these beds is 

 required. First, if we look to see what species there are in common, 

 we shall find that, although not numerous, they are well marked 

 and characteristic. The folloA^ing shells are common and special to 

 this zone both in France and England : — 



Cerithium mutabile, Lam. Pectunculus deletus, Brand. 



Chama squamosa, Brand. Trochus monilifer, Lam. 



Couus scabriusculus, Sow. patellatus, Desk. 



Lucina ambigua, Defr. Venericardia cor-aviura, Lam. 

 Menardi, Desk. Voluta ambio^ua, Brand. 



Modiola seminuda, Desk. . athleta, Sow. 



Ohva Branderi, Soiv. dejjauperata, Sow. 



and I think, probably, — 



Volvaria acutiuscula, Sow.l Corbula umbonella. Desk. 



Whilst the following species, although extending into other beds in 

 England, are peculiar to the Sables moyens in France : — 



Cancellaria evulsa, Brand. ? Murex asper. Brand. 



Clavagella coronata, Desk. Voluta digitaliua. Lam. 



If, further, we measure the distinctiveness of each fauna in its own 

 area, we shall find that the two deposits are distinguished by a 

 nearly like amount of difference. Thus the relative proportion of 

 all the species ranging from the lower to the upper deposits in each 

 country respectively is in the following ratio : — 



Barton 30 Sables moyens. ... 35 



Bracklesham .... 100 Calcaire grossier . . 100 



Whilst if we take the per-centage in descending order, we have : 



Barton 100 Sables moyens. ... 100 



Bracklesham .... 45 Calcaire grossier . . 50 



These are only submitted as approximate numbers *. 



Belgium. — Above the Bruxellian System in Belgium there are 

 20 to 30 feet or more of green and yellow sands occasionally pebbly 

 at base. M. Dnmont has termed these the Laekenian System, and 

 considers them, as does Sir C. Lyell, to be the equivalent of the 

 Barton Clay. There appears a difficult v connected with such a cor- 

 relation, inasmuch as the proportions of Barton and Bracklesham 



* The amount of difference between the two French deposits was first distinctly 

 noticed by M. D'Archiac in 1837 (Bull. Soc. Geol.). Out of 320 species of 

 " Sables moyens " shells, he estimated that 166 ranged from strata lower in the 

 Eocene series, and that 154 were peculiar to that formation. 



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