228 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Or, taking the total number of species in these three French deposits * 

 respectively and jointly with those of the London Clay, as 376, 571, 

 and 863, then the numbers common to them severally show the 

 following per-centagef : — 



London Clay, Glauconie London Clay and Lits London Clay and Cal- 

 Inferieure, and Lignites. Coquilliers. caire grossier. 



4-5 4-4 1-6 



The proportion of common species appears therefore in any case 

 to be remarkably small, and, palseontologically, the London Clay 

 shows nearly the same relation to the Lits Coquilliers that it does to 

 the Glauconie Inferieure — one evidently closer than with the Calcaire 

 grossier. It may be suggested that these conclusions do not accord 

 wdth those other conclusions upon which the correlation of the Brack- 

 lesham Sands and the Calcaire grossier has been previously esta- 

 blished, inasmuch as whilst we have here a per-centage of only 1*6 

 species common to the Calcaire grossier and the London Clay, I have 

 on a former occasion shown that there are 9 '4 species common to the 

 two English deposits — a proportion greater even than that which holds 

 between either the Glauconie Inferieure or the Lits Coquilliers and the 

 London Clay, both of which, I conclude, will prove nearer equivalents. 



To comprehend this anomaly, we must take each separate area on 

 its own base, and determine the lapse of time by the successive changes 

 there introduced ; in each considering the progression of time as parallel 

 and independent movements. These circumscribed centres may show 

 great irregularities and hiatuses ; the adjacent structures may at 

 one time exhibit close relations with one another, whilst at interme- 

 diate and still synchronous periods, their aberration, arising in most 

 cases from the independence of their zoological provinces, may be 

 such as to produce dissimilarities greater than those which prevail 

 between successive periods in each local centre itself. There are 

 generally, however, at intervals, limits to these variations, which may 

 serve to indicate the exceptional causes to which the latter are owicg. 

 These limits occur whenever two adjacent centres become so connected 

 that like or nearly allied hydrographical conditions prevail over both 

 areas at the same time so as to favour a community in the faunas. 

 We are then furnished with a definite base-line to which we may 

 safely refer all subordinate and intermediate changes 



We have every reason to believe that a close connexion of this 

 nature existed at the Calcaire grossier period between those beds in 

 France and the Bracklesham series in England;]:. There are, it 

 is true, not unimportant dissimilarities, but these are the perma- 

 nent dissimilarities — those which belong to ordinary and constant 

 geographical laws, and only modify without essentially impairing the 

 zoological resemblances. They are dissimilarities which have grovm 



* In this I take the number of Calcaire grossier species given by M. Graves, 

 his list being the largest and most complete. 



t Looking at the French groups only, the numbers would be — 

 11-2 7-2 205 



X Commencing with lower Bracklesham or Bagshot Sands, and attaining its full 

 force at the time of the Glauconie Grossiere and Lower Calcaire Grossier. 



