82 INVERTEBRATE PALAEONTOLOGY 



marine types must be ascribed to confusion of faunas by 

 transporting agents. Further, discovery of Graptolitefi 

 in black shales whose abysmal origin seems probable is 

 no proof of the benthic habitat of those organisms. 

 Their occurrence in such deposits is probably due to the 

 sinking of planktonic surface-dwellers when buoyancy 

 was lost. 



(I) TERRESTRIAL FAUNAS 



The rarity of fossils in terrestrial deposits is par- 

 ticularly marked in respect of Invertebrates. Only two 

 phyla, the Mollusca and Arthropoda, are generally avail- 

 able for fossilization on land, and their representatives 

 are confined to relatively few types endowed with some- 

 what delicate shells. The dominant terrestrial members 

 of the two phyla are the Gastropoda and Insecta ; in the 

 case of the former an exiguous proportion only are 

 Pulmonate. The average " snail " has a less massive 

 shell than a marine Gastropod ; and " slugs " secrete so 

 little calcareous matter that preservation and discovery 

 of their remains must always be doubtful. Except in 

 the uppermost layers of the soil, dead snail-shells are 

 rarely found, owing to the ready access of bacterial and 

 aqueous agents of destruction. They may occur in con- 

 siderable numbers in fresh-water deposits whither they 

 have been drifted by streams ; but in such cases they 

 have passed out of their true province, and serve only 

 to indicate the proximity of land to the place of their 

 burial. To their extreme rarity in sub-aerial deposits a 

 further contributory factor exists in the usually desert 

 origin of geologically permanent terrestrial accumula- 

 tions. Moreover, in rocks of considerable antiquity, it is 

 difficult to be certain of the " pulmonate " character of a 

 Gastropod, since the outward form of the shell gives 

 little guidance towards determination of" soft " anatomy. 



