THE MAGAZINE 



OF 



NATURAL HISTORY 



MARCH, 1838. 



Art. I. Considerations upon the position in the Tertiary System, 

 to which the Faluns of the Loire, and the Crag of England, ought 

 to be referred; and upon the difficulty of determining their rela- 

 tive age, solely by the law of the proportional number of fossil spe- 

 cies, analogous to species now in existence.* By M. Desnoyers. 



J_ his law, which consists in regarding a tertiary formation as 

 so much the more recent, in proportion as it contains a great- 

 er number of species of shells analogous to those now exist- 

 ing, all other characters being put aside, may be excellent in 

 itself, (as M. Deshayes has shewn, who has drawn from it 

 the most positive inferences) especially when it is strengthen- 

 ed by geological considerations, derived, either from the more 

 or less direct superposition of beds, or from the geographical 

 relations of basins : it is, however, of very difficult application, 

 and may lead into serious errors, or, at least, into contradic- 

 tory results, in a case where the fossils of similar strata are 

 examined by conchologists, who form different estimates of 

 the value of characters, in the discrimination of species ; or, 

 more particularly, when it is endeavoured to fix the relative 

 age of many strata, according to numerical results, obtained 

 separately by observers who do not agree upon the essential- 

 ly distinguishing specific marks. 



Now we know how far naturalists are from a general un- 

 derstanding, with respect to the extremely nice question, as 

 to the limits of possible variations occurring at particular pe- 

 riods of growth, or in consequence of the influence of this or 

 that situation, and other local circumstances ; not only as re- 

 spects the Mollusca, but in all the other branches of zoology. 



* From the ' Bulletin de la Societe Geologique de France ; ' April 3rd, 

 1837. 



Vol. II. — No. 15. n. s. k 



