GEOLOGICAL DISTRIBUTION OF MOLLUSCA. 265 



Vallonia pulchella. 96 The various hypsometric zones that have been 

 established by conchologists differ at almost all parts of the earth's 

 surface, and are of but local import. The shells of the more 

 elevated mountain -summits are many of them, or mostly, of types 

 which are found at lower levels in regions of reduced average 

 annual temperature, following the well-known law of climatic 

 dispersion which we recognise among plants. Vertigo alpestris, 

 an inhabitant of Scandinavia, reappears in the Alps of Switzerland, 

 although completely wanting in the intermediate regions; and, 

 similarly, many Alpine summits hold identical or representative 

 species which are wanting in the connecting lowlands. 



Geological Distribution, The most salient fact that presents 

 itself in connection with the past distribution of the Mollusca is 

 the reversed order to what might have been expected of the suc- 

 cessive development of its primary classes. Thus, almost every- 

 where, the Cephalophora, or head-bearing mollusks, antedate by 

 one full period the Acephala, or headless forms, which indisput- 

 ably represent a lower grade of organism ; and among themselves 

 the first to attain a maximum development are the cuttle-fishes 

 (Cephalopoda), which are structurally the highest. But two species 

 of this group an Orthoceras and a Cyrtoceras are positively known 

 from the Cambrian formation, while in 1868 Bigsby enumerated no 

 less than one hundred and thirteen species of Gasteropoda as belong- 

 ing to the same period of time. On the other hand, the total 

 number of gasteropod species credited by the same author to the 

 Silurian deposits is about eight hundred, whereas Barrande has 

 described upwards of eleven hundred species of Nautilidae from 

 the Upper Silurian deposits of Bohemia alone. As has already been 

 intimated, there is a marked deficiency of Cambrian lamellibranchs, 

 and even in the Silurian formation the number of species is com- 

 paratively limited. Bigsby, in 1808, enumerated, all in all, some 

 six hundred and thirty-six species, or but little more than one-half 

 the number of Upper Silurian cephalopods of the Bohemian basin. 

 The complete differentiation which the different classes of the 

 Mollusca had already attained in the Silurian period argues for a 

 great antiquity beyond that period of the members of this group. 

 To what extent the time measured by the Cambrian period, and 

 the interval intervening between it and the Silurian, may have been 

 effective in bringing about the various changes, cannot be at pres- 



