92: PEOP. J. PKESTWICH ON THE COEKELATIODf OE THE 



tiaries both in England and France, it constantly happens that more 

 than one group or zone is exposed in the same section, so that there is 

 frequently a risk of the specimens getting mixed, or being referred 

 to a wrong zone. It is further to be observed that the Anglo- 

 Belgian Basin is separated from the Paris Basin by a wide plateau of 

 Chalk, and, although there are connecting links left here and there 

 upon the Chalk hills, these masses are generally isolated, and this, 

 together with changes of conditions and distance, frequently renders 

 it difficult to follow the sequence of the zones, so that, in this case, 

 beds belonging to different levels may have been referred to the same 

 geological horizon, and their fossils have passed current under the 

 name of one dominant zone. Thus the Sands of St. Omer and La 

 Pere, which belong rightly to the Thanet Sands and Lower Lande- 

 nian, are held to be synchronous with those of Bracheux, and it has 

 been customary to speak of the fossils of those beds indifferently as 

 Bracheux-Sands species. Somewhat similar circumstances have 

 occasionally arisen with respect to the fossils of the Oldhaven and 

 Basement-beds, and of the Woolwich Beds and Thanet Sands in 

 East Kent. 



To avoid any confusion from these sources of error, I have in the 

 following lists confined myseK, as far as possible, to the one locality 

 where one zone only is exposed, and which may be regarded as 

 affording a true type of the zone in question. Thus Pegwell Bay 

 offers such a section for the Thanet Sands, and the Butte de la 

 Justice near Beauvais offers such another for the Bracheux Sands. 

 In the same way, I take as a tj^pe of the Oldhaven Beds the bed 

 immediately beneath the London Clay at Oldhaven Gap, which I 

 originally designated the Basement-bed of the London Clay, and I 

 have eliminated the Sundridge and Charlton fossils, which my friend 

 Mr. Whitaker places with the Oldhaven. 



To resume with the Thanet Sands and Lower Landenian, the 

 described Mollusca of the former amount at PegweU Bay only to 27 

 species, whereas, taking the Tufeau de Lincent as the type of the 

 latter, the number of known shells there recorded by Dumont, JN'yst, 

 and Dewalque amounted to 33, and to these M. Vincent has now 

 added 31, most of them new and peculiar species, and 3-i others 

 which he has onty named, but not described. It is not, however, 

 necessary to say more about these beds than that they are in both 

 countries characterized by such common species as Cy]^rina Morrisii, 

 Cucullcea crassatma, Plioladomya KonincHi, P. cuneata^ Sccdaria 

 Boiuerlanhii, &c. The following list of the Pegwell-Eay fossils 

 will show that a considerable proportion of the species are peculiar 

 to that zone * ; and while a certain number of characteristic species 

 are common to the Thanet Sands and Lower Landenian, each group 

 possesses a certain proportion of peculiar species. 



* The relative proportion of the Lower Landenian fossils to those of the 

 Bracheux is made larger by M. Vincent ; but this arises from embracing under 

 the designation of Bracheux Sands, not only those of the Beauvais district, but 

 of various other localities in the Departments of the Aisne, Marne, and Somme, 

 which are habitually referred to the Bracheux Sands. Eliminating these, there 

 are 66 peculiar species out of M. Vincent's total of 97. 



