430 MANUAL OF THE MOLLUSCA. 



average extent, enabling tliem to acquire a wide range, notwithstanding llieir 

 tardy migrations. 



But when we compare the estimated rate of change in physical geogra- 

 phy with the duration of genera and families of shells, we not only find 

 ample time for their diffusion by laud or sea over large portions of the world, 

 but we may perceive that such transferences of the scene of creation must 

 have become inevitable. 



Method of Geological Investigation. — In whatever way geological history 

 is written, its original investigators have only one method of proceeding — 

 from the known to the unknown — or backwards in the course of time. 



The newest and most superficial deposits contain the remains of man and 

 his works, and the animals he has introduced. 



Those of pre-historic date, but still very modern, contain shells, &c., of 

 recent species, but in proportions different from those which now prevail, 

 (p. 384, 387). Some of the species m.ay be extinct in the immediate neigh- 

 bourhood of the deposits, but still living at a distance. 



In the harbour of New Bedford are colonies of dead shells of the Pholas 

 costata, a species living on the coast of the Southern States. At Brackle- 

 sham, Sussex, there is a raised sea-bed con' .ining 35 species of sea-shells 

 living on the same coast, and 2 no longer living there, viz. — Pecten pohjmor- 

 phus, a Mediterranean shell ; and Ltdraria rugosa, still found on the coasts 

 of Portugal and Mogador. 



Tertiary Age. — If any distinction is to be made between " Tertiary " and 

 " Post-tertiary " strata, the former term should be restricted to those deposits 

 which contain some extinct species. And the newest of these, in Britain, 

 contain an assemblage of Northern shells. Prof. Forbes has published a list 

 of 124 species of shells from these " Glacial beds," nearly all of which are 

 now existing in British seas.* 



In most of the localities for glacial shells, the species are all recent ; but 

 at Bridlington, Yorkshire, and in the Norwich Crag, a few extinct species 

 are found, (e. g. micida Cohboldice, PI. ]7, f. 18.) At Chillesford, Suffolk, 

 Toldia arctica and inyalis occur of large size and in excellent preservation, 

 with numerous specimens of 3ha truncata, erect as they lived, in the muddy 

 sea-bed. Troplion scalariforme, Admete viridala., Scalaria grcenlandica, 

 and Natica grcenlandica, also occur in the Norwich Crag ; and Astarte bore- 

 alts, with several arctic forms of Tellina, are amongst the commonest shells, 

 and frequently occur in pairs, or with their ligament preserved; the deposit 

 is extensively quarried for shell-sand. 



Raised sea-beds with Arctic shells at Uddevalla in Sweden, have been 

 repeatedly noticed ever since the time of Linnreus. Captain Bayfield disco- 



* The species which have retired further north are marked (**) in the preceding 

 Arctic List, p. 355. 



