852 M. L. Agassiz on the Primitive Diversity and 



variety of types, than are found in the tertiary of Paris ; for the 

 whole basin of the Red Sea has hitherto yielded only 400 species 

 of shells. Let us finally take the most accurate survey of this 

 kind that we have of any shore^ that of Panama by Prot'. Adams, 

 extending over 50^ of latitude, 28° N. of the equator and 22° S. 

 of it, including the most favourable localities for the growth of 

 shells in the Pacific under the tropics, and yet we shall find his 

 list exceeding but little the number of 500 species. In this 

 instance, again, we find that the advantage in number and variety 

 is in favour of the tertiary period, and not of the present age. 

 If a different result has been obtained by the estimates made 

 before this, it is owing to the circumstance that the /osszY^ known 

 from a few localities within narrow geographical limits were com- 

 pared with the living species known to occur upon the whole sur- 

 face of the globe. But let us trace these comparisons through 

 other geological periods, with reference to other classes also, and 

 we shall find in every instance similar results. The tertiary fos- 

 sils of Bordeaux, though less numerous in species than those of 

 the eocene in the vicinity of Paris, will compare with any local 

 fauna of the present period as favourably for variety and number 

 of species as those of the lower tertiaries. This may be said with 

 the same certainty of the tertiary shells of the Subapennine hills^ 

 or of those of the English Crag, of which we now possess a very 

 complete list. 



If from the tertiary periods we pass down to the cretaceous, 

 do we not find in the deposits of Maestricht, or in those of the 

 age of the white chalk, a number and variety of shells as great 

 as that which may be found on any shore or in any circumscribed 

 marine basin of an extent at all comparable with that of the cre- 

 taceous beds within similar limits ? Do we not find in the lower 

 cretaceous strata, such as the green sand or the Neocomian, other 

 assemblages of the remains of mollusks, which in number and 

 variety are not inferior to those of the white chalk ? The oolitic 

 series, again, will stand a similar comparison quite as well. We 

 need not even take the whole group of those deposits, but con- 

 sider each subdivision of the Jurassic period by itself, and still 

 we find in every one, local faunae of mollusks. assuming, of 

 course, a different character from those of the cretaceous or 

 tertiary, but nevertheless sufficiently diversified to admit of an 

 estimate, as advantageous, with respect to the points under con- 

 sideration, and to the local faunse of the present day, as the 

 cretaceous assemblages of fossils, or those of the tertiary period. 

 Of course, in accordance with the peculiar character of the age, 

 different families prevail in these different periods ; the Cepha- 

 lopoda are extremely numerous, and surprisingly diversified 

 during the cretaceous and oolitic periods; while they dwindle 



