Vlll PREFACE. 



obtained chiefly from the Italian strata; and as I had 

 already conceived the idea of classing the different 

 tertiary groups, by reference to the proportional 

 number of recent species found fossil in each, I was 

 at pains to learn what number Signor Bonelli had 

 identified with living species, and the degree of 

 precision with which such identifications could be 

 made. With a view of illustrating this point, he 

 showed us suites of shells common to the Sub- 

 apennine beds and to the Mediterranean, pointing 

 out that in some instances not only the ordinary type 

 of the species, but even the different varieties had 

 their counterparts both in the fossil and recent series. 

 The same naturalist informed us that the fossil shells 

 of the hill of the Superga, at Turin, differed as a 

 group from those of Parma and other localities of 

 the Subapennine beds of northern Italy; and, on the 

 other hand, that the characteristic shells of the 

 Superga agreed with the species found at Bordeaux 

 and other parts of the South of France. 



I was the more struck with this remark, as Mr. Mur- 

 chison and myself had already inferred that the highly- 

 inclined strata of the Valley of the Bormida, which 

 agree with those of the Superga, were older than 

 the more horizontal Subapennine marls, by which the 

 plains of the Tanaro and the Po are skirted. 



When we had explored some parts of the Vicentin 

 together, Mr. Murchison re-crossed the Alps, while I 

 directed my course to the south of Italy, first staying 



