246 PALAEONTOLOGY. 



strata were formed. We are indebted to Professor Grant* 

 for the following summary of this class : 



The species of gasteropods are much less abundant in 

 the ancient grauwacke limestones than those of bivalved 

 mollusca ; only about seventy species of the former having 

 been yet identified in the strata of that epoch, and a quarter 

 of these belong to the extinct genus euomphalus, which ceases 

 with the carboniferous rocks. Most of the species, however, 

 observed in these ancient grauwacke formations are referred 

 to existing genera, as turritella, of which about ten species 

 occur ; turbo, six species ; buccinum, patella, delphinula, five 

 each ; nerita, pileopsis, trochus, and phasianella, three species 

 each. The gasteropodous genera continue in nearly the same 

 proportions in the carboniferous strata, and several even of 

 the species are identical in the two formations ; as, pileopsis 

 vetusta, melania constricta, euomphalus nodosus and citillus, 

 turbo tiara, rotella helicinceformis, buccinum acutum, and 

 turritella prisca. The euomphalus, turritella, trochus, turbo, 

 and nerita are still the most abundant genera of the carboni- 

 ferous gasteropods ; and a few species only of this class have 

 been recognised in the coal measures, or in the new red sand- 

 stone group of rocks ; but in the oolites more than a 

 hundred and thirty species of univalves occur, which belong 

 chiefly to the genera trochus, patella, turbo, nerincea, tur- 

 ritella, actceon, melania, natica, cirrus, pleurotomaria, nerita, 

 cerithium, rostellaria, pterocera, and trochotoma. 



In the cretaceous rocks, not half of the number of gastero- 

 pods occur which are found in the oolitic formations, and 

 they belong, for the most part, to the recent genera, trochus, 

 rostellaria, turbo, cirrus, vermetus, auricula, and dentalium. 

 But so abundant are these animals in the marine deposits of 

 the tertiary epoch, that more fossil species belonging to a 

 single genus are found in them than belong to all the cre- 

 taceous genera of gasteropods taken collectively ; thus, two 

 hundred and twenty fossil species of cerithium are met with 

 in the tertiary strata a number nearly treble that of the 

 known existing species of that genus ; one hundred and fifty- 

 six species of_pleurotoma are found in the tertiary formations; 

 one hundred and eleven species of fusus, and ninety-five 



* Thomson's Annual, p. 247. 



