1852.] LYELL — BELGIAN TERTIARY FORMATIONS. 323 



Eocene strata, whether at Rupelmonde or Kleyn Spawen. In that 

 country, as in England and France, they seem to characterize the 

 Middle Eocene series, and not to occur in the Upper or Lower 

 Eocene ; a result corresponding to that at which M. d' Archiac had 

 previously arrived in France. 



The proportion of recent species in the 201 fossil MoUusca of the 

 Limburg beds has still to be considered. I may safely affirm that it 

 is not greater than in older or universally acknowledged Eocene for- 

 mations. The following fossils have been supposed to agree with 

 living shells : — 



1. Cyprina islandica ?, var. 5. Limneus fabulum ? 



2. Rissoina Nystii. 6. Solen ensis, var. ? 



3. Rissoa violacea ? 7. Ostrea cochlear. 



E. plicata. 



4. Planorbis corneus ? 



P. rotundatus ? 



Judging from one valve of the Cyprina above mentioned, given to 

 me by M. Bosquet, Professor Forbes believes the shell to differ from 

 the living species ; the muscular impression being larger, and the 

 pallial impression not similar. Rissoa plicata, Desh., appeared to 

 M. Bosquet to agree perfectly with U. violacea, Frem. and Desm., a 

 Mediterranean shell, but Prof. E. Forbes, after carefully comparing 

 them, is not satisfied with the identification ; and he remarks, in regard 

 to the genera Rissoina and Rissoa, that there is so great a want of 

 unanimity among the best naturalists as to the value of the specific 

 characters of some of the commonest living species, such as Rissoa 

 balthica, that it would be dangerous to attempt to identify fossils in , 

 these genera with living molluscs, when they occur as members of a 

 fauna decidedly extinct. The same remark will apply to Limneus, 

 Planorbis, and Ostrea, I do not possess the means of comparing the 

 Solen alluded to by M. Nyst. Two or three of the Entomostraca, as 

 Bairdia lithodomoides and Cytheridea Mulleri, are pronounced, on 

 the high authority of M. Bosquet, to be quite undistinguishable from 

 living species. Such identifications do not affect the data on which 

 my original nomenclature of tertiary classification was founded, as I 

 confined myself exclusively to the fossil moUusca. 



With regard to the faluns of Touraine, although containing more 

 than 300 species of shells, there are scarcely any species identical 

 with those of the Limburg ; so that the greater analogy of these 

 last with the Eocene tjrpe, than with that which I have always con- 

 sidered as Miocene, is striking. 



§ 7. On the Middle Eocene Strata of Belgium and French Flanders, 

 or the "Nummulitic Eocene " (E. 1, 2, 3, Table I. p. 279). ^S'y- 

 stemes Laekenien, Bruxellien et Tpresien, etage superieur, of 

 M. Dumont. 



The group of tertiary strata which we meet with next in the de- 

 scending order in Belgium (comprising the " Systemes Laekenien, 

 Bruxellien et Ypresien " of M. Dumont) corresponds most nearly, if 



