is? MANUAL OF THE MOLLUSCA. 
The shells of the newer tertiaries are always identical, at least 
generically, with those of the nearest coasts. Thus, in Pata- 
gonia are found species of Trophon, Orepidula, Monoceros, 
Pseudoliva, Voluta, Oliva, Crassatella, and Solenella. The ter- 
tiaries of the United States contain species of Fulgur, Mercenarta, 
and Gnathodon. The miocene shells of St. Domingo appear at 
first sight to be all of recent species, but on comparison prove 
to be mostly distinct. 
The proportion of extinct species in the Pliocene tertiary 
varies from 1—50 per cent. If a deposit contains more than 
50 per cent. of extinct species it is referred to the Miocene 
period; and this test is particularly valuable since the modern 
deposits are often isolated, and frequently no assistance can be 
derived from superposition, or even from identity of species. 
In the Hocene tertiaries we perceive the ‘‘dawn”’ of the present 
order of things. All, or very nearly all, the species are different, 
but a large proportion of the genera are still existing, though 
not always in the seas nearest to the localities where they occur 
fossil. 
Thus in the London clay are found—Rostellaria, Oliva, Ancil- 
laria, and Vulsella, genera still living in the Red Sea; and many 
species of Nautilus, Rimella, Seraphs, Conus, Mitra, Pyrula, 
Phorus, Liotia, Cardilia—genera characteristic cf the Indian 
Ocean; Cyprovula, Typhis, and Votlutilithes, now living at the 
Cape; Clavella, at the Marquesas, and Pseudoliva, Trochita, and 
species of Murex, whose recent analogues are found on the 
western shores of South America. 
The freshwater shells of this period are Old World forms: 
Melanopsis, Potamides, Lampania, Melanatria, and Nematura ; 
whilst the land-shells form a group quite American in character 
—large species of Glandina and Bulimus (with reflected lip) 
Megalomastoma (mumia), a Cyclotus (with its operculum) like C. 
Jamaicensis, and the little Helix labyrinthicus. 
Secondary Age.—In none of the older strata do we find idica- 
tions of a warmer climate having prevailed, in the latitude of 
England, than that which marks the period of the London clay. 
And this is not more than can be accounted for by such a cause 
as the flow of an equatorial current from the direction of the 
Red Sea, until arrested by a continent to the south-west, as 
supposed by Mr. Prestwich, in the region of the Azores. 
Some indications exist of a more moderate climate haying 
obtained in the north polar regions; for remains of the Ichthyo- 
saurus were found at Exmouth Island, the farthest point reached 
by Sir E. Belcher’s expedition. 
