GEOLOGICAL MEMOIRS. 



1 . Number of Molluscous species at present compared with the number 

 during the Tertiary period. 



In the tertiary strata of South Italy we find scarcely any fossil re- 

 mains but those of marine animals, and of these it is not to be expected 

 that any indications of the naked mollusca should be preserved. 

 Omitting, therefore, the latter, and also the land and freshwater 

 shells, from the list of existing Mollusca, we have the following table 

 of the numerical relations between the tertiary and existing period :— 





Existing period. 



Tertiary period. 



Conchifera 



188 sp. or 35 per cent. 



10 „ 2 „ 



11 „ 2 „ 

 313 „ 58 „ 



15 „ 3 „ 



537 species. 



231 sp. or 40 per cent. 

 13 „ 2i „ 



5 „ 1 „ 

 322 „ 56 „ 



5 „ 1 „ 



b7Q species. 





Pteropoda 



Gasteropoda (with shells) ... 

 Cirripeda 





The sea therefore was somewhat richer during the tertiary period 

 than it is at present ; and it must be remembered that the number 

 of fossil species is likely to be increased by future researches to a 

 much greater extent than that of existing species ; but, on the other 

 hand, it must also be borne in mind that the tertiary period involves 

 a much longer lapse of time, during which many species have died 

 out and new ones been introduced, and it is therefore not improba- 

 ble that with respect to number of species there is no marked dif- 

 ference between the past and the present. 



The relative proportion of the different groups was formerly 

 somewhat different from that which now obtains, as will be observed 

 by looking at the per-centage in the foregoing table, and it thence 

 appears that the Conchifera and Brachiopoda formerly preponde- 

 rated, while the Gasteropoda and Cirripedes are more abundant at 

 present. I think we may hence conclude that the extent of coast 

 was less during the tertiary period than it is at present, and that the 

 bed of the sea recently elevated into land consisted chiefly of shal- 

 lows. 



2. Proportion of extinct to existing species. 



Among the 537 existing species of marine shells, I have at pre- 

 sent only met with the following in a fossil state. They amount to 

 169 in number, being somewhat less than one-third of the whole. 



Conchifera, 



Clavagella balanorum, Scac. 



angulata, Ph. 

 Teredo navalis, L. 



Bruguieri, D. Ch. 



palmulata, D. Ch. 

 Pholas Candida, L. 

 Solen legumen, L. 

 Panopaea Aldi-ovandi, Men. 

 Scrobicularia piperata, Gm. 



Scrobicularia Cottardi, Pay. 

 Erycina ovata, Ph. 

 Bornia serainulum, Ph. 

 Solenomya mediterranea, Lam. 

 Corbula revoluta, Broc. 

 Pandora flexuosa, Sow. 

 Thracia ovahs, Ph. 



fabula, Ph. 

 Galeomma Turtoni, Sow. 



