100 GEOLOGICAL MEMOIRS. 



Great Britain therefore, compared with Sicily, gives — 



Marine Bivalves 



Absolute No. of 

 Species.' 



No. of Species 

 common. 



Proportion of Species 

 common. 



British. 

 198 



Sicilian. 

 188 



84 



or 42 per ct. Brit. 



45 Sicil. 



Fresh-water do. 



10 



11 



6 



60 



54 „ 



Brachiopoda 

 Naked marine Gas- "1 

 teropoda J 

 Conchiferous do. do. 



5 



20 



191 



10 



54 

 313 



2 



7 



56 



35 

 29 



13 „ 



18 „ 



Land and fresh- "1 

 water do. J 



93 



186 



45 



48 



^5 „ 



Cephalopoda 



7 



15 



5 







It appears from the above table that (with the exception 

 of the less numerous and therefore for purposes of com- 

 parison more doubtful Cephalopoda) the Bivalves exhibit the 

 maximum analogy between this part of the faunas of the two 

 countries ; that next to them come the land and fresh-water 

 Gasteropoda, and last of all the marine Gasteropoda. It results 

 therefore that the geographical extension of the Mollusca, bears 

 an inverse ratio to their powers of locomotioti, and this is con- 

 firmed by the other comparisons and analogies about to be 

 given. But I must not omit to remark that in the above com- 

 parison of the land and fresh- water Gasteropoda, there appear 

 several species which I have not myself seen, but have included 

 on the authority of the Sicilian conchologists, and that Professor 

 Maravigna reproaches his countrymen with often giving foreign for 

 indigenous species. 



It is however to be observed, that not only is the number of 

 Mediterranean species absolutely greater than that of Great 

 Britain, but that the number of the genera, and also the extent and 

 variety of types, is also much more considerable ; and although, 

 in my opinion, the boundary lines defining many genera are still 

 vague and unphilosophical, I may, notwithstanding, give as sufficient 

 proof of this difference the following tables : — 



List of South Italia?i Genera not represented in Great Britain. 



Clavagella 



Idalia 



Notarchus 



Fasciolaria 



Solenomya 



Diphyllidia 



Onchidium 



Tritonium 



Scacchia 



Pleurobranchaea 



Sigaretus Lam. non 



Cuv. Ranella 



Cardita 



Umbrella 



*"Haliotis L. 



Cassidaria 



Chama 



Tvlodina 



Siliquaria 



Columbella 



Spondylus 



Crepidula 



Fossarus 



Mitra 



Thecidia 



Acera 



Solarium 



Ovula 



Thetis 



Gasteropteron 



Cancellaria 



Conus 



* Haliotis is not met with on the actual coast of Great Britain, but belongs 

 rather to the Channel Islands. 



