132 MANUAI OP THE MOLLTJSCA. 



The shells of the newer tertiaries are always identical, at least 

 genericaUy, with those of the nearest coasts. Thus, in Pata- 

 gonia are found species of Troplion, Crepidula, Monoceros, 

 PseudoUva, Valuta, OJiva, Crassatella, and Solenella. The ter- 

 tiaries of the United States contain species of Fulgur, Mercenaria, 

 and Gnathodon. The miocene shells of St. Domingo appear at 

 first sight to he 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 Eocene 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 — Bosfellaria, Oliva, Ancil- 

 laria, and Vulsella, genera still living in the Eed Sea ; and many 

 species of Nautilus, Bimella, Seraphs, Comes, Mitra, Pyrula, 

 Pliorus, Liotia, Cardilia — genera characteristic of the Indian 

 Ocean ; Cyprovula, Typhis, and Volutilithes, now living at the 

 Cape ; Clavella, at the Marquesas, and PseudoUva, 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 (jnumia), a Cyclotus (with its operculum) like C, 

 Jamaicensis, and the little Helix lahyrinthicus. 



Secondary Age, — In none of the older strata do we find indica- 

 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 

 Eed 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 having 

 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. 



