ex 



INTRODUCTION. [cH. 



are merely what logicians call '' accidents/^ All the earth 

 (even granite and felspar) is said to contain calcareous 

 matter, although the proportion is of course greater in 

 some formations than in others. The case of two com- 

 mon and conspicuous land-shells occurs to me with 

 reference to this question. Helix lapicida is directed 

 by Forbes and Hanley^ " to be sought for in limestone 

 and chalky districts." It is common, however, in the 

 trap formation of the Lower Harz, as well as in the 

 molasse of Switzerland. According to Moquin-Tan- 

 don the Cyclostomata " aiment surtout les terrains cal- 

 cairesf;" but our only species (C elegans) is tolerably 

 abundant in Jersey, where there are no calcareous strata. 

 It would be easy to adduce many similar instances to 

 prove that the habitat of Mollusca is not so restricted, 

 as has been stated, in their geological relations. But 

 there is no doubt that, with regard to land- shells, 

 both granite and peat (which are at the opposite ends 

 of the geological scale) are equally unfavourable to mol- 

 luscan Hfe; because the former is not easily disinte- 

 grated and converted into mould, so as to support vege- 

 tation, and the latter, being chiefly composed of the bog- 

 moss (or Sphagnum), is either innutritions or distasteful 

 to snails. The same observation applies to fir- woods, 

 which do not appear to be inhabited by the Mollusca. 

 With respect to the marine Mollusca, it may be ob- 

 served that the phytophagous kinds will be found in 

 abundance wherever sea-weeds flourish, and that in the 

 deeper zones of the sea, in which such vegetation is 

 absent, an ample supply of animal food is not wanting. 

 But the substance of molluscous shells undoubtedly de- 

 pends on the nature of the soil ; and carbonate of lime 

 seems to be as necessary to most snails for the secretion 

 * Brit. Moll. iv. p. 66. t Hist. Moll. Fr. t. ii. p. 492. 



