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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 
