1852.] LYELL BELGIAN TERTIARY FORMATIONS. 303 



nearly allied to undescribed London Clay species, come so near to 

 them, that by some conchologists they might be thought identical. 

 The affinity on the whole is more with the Barton Clay than with 

 older members of the Eocene series. The general aspect of this 

 fauna is so decidedly Eocene, that I am not surprised that M. Ar- 

 chiac, having no means near Antwerp of determining the relative 

 position of the Rupelmonde clay, and knowing that MM. de Koninck 

 and Nyst had identified one-fourth of the shells with English Eocene 

 species, persisted even in 1848 in believing that it was part of the 

 London Clay proper, which it resembles in mineral character, in the 

 colour of its clay, its contained septaria, and its nodules of pyrites*. 



The remains of fossil fish from Rupelmonde, as will be seen by the 

 list, consist partly of London Clay and Calcaire grossier species, such 

 as Otodus obliquus, Lamna elegans, and L. compressa, and partly of 

 species cited by Agassiz as from the " molasse " of Switzerland. 



Of the 28 species of shells which I myself obtained at Rupelmonde 

 and the vicinity, the most abundant by far was Nucula Beshayesiana, 

 and after it Fusus multisulcatus. The occurrence of seven species of 

 Pleurotoma, some of them very common, is also a striking character, 

 from which, and from a consideration of the whole of the data com- 

 prised in the foregoing list, my friend Professor E. Forbes infers that 

 the clay of Rupelmonde was deposited in a sea at about the junction 

 of his perilittoral and median zones of depth, or between 15 and 25 

 fathoms, probably nearer to 15 than to 25 f. 



It will be necessary to defer the consideration of the true age of 

 the Rupelmonde Clay, until I have described the tertiary strata of 

 the province of Limburg. 



2. Tertiar]) Strata in the neighbourhood of Kleyn Spawen, near 

 Maestricht. Limburg Tertiaries. 



I have already alluded to the services rendered by MM. de Ko- 

 ninck and Nyst to the palaeontology of Belgium by their description 

 of the Rupelmonde fossils. The greater part of the Kleyn Spawen 

 MoUusca have also been figured and described by M. Nyst, while M. 

 Bosquet has recently given us an able account of the Entomostracous 

 Crustaceans of the same district. The last-mentioned naturahst has 



* Archiac, Hist, des Progr. torn. ii. p. 498. 



f As Professor Forbes has recently modified his nomenclature of the zones of 

 depth, in order to render his terms more appUcable to the seas of all climates and 

 all parts of the globe, I give the subjoined explanation of his new names as used 

 in this memoir : — 



Marine Zones of Depth, according to Prof. E. Forbes, 1852. 

 Names. Depths. 



Littoral zone Between tide-marks. 



Perilittoral zone { {|Pf/^ ''' ^^"l!"^';!^!'. .'^!!!!!'" } « *« ^^ ^^*^^^«- 



,, ,. , r^ 11- N f Upper Median 15 to 30 „ 



Median (or Corallme) zone JL^ier Median 30 to 50 „ 



Infra-median zone 15 to 100 ,, 



Abyssal zone 100 to „ 



X 2 



