time, both in depth and in climatic range, and it is the same 



now 



CoNCHiFEBA become more and more numerous as time 

 advances, and relatively are more abundant now. 



The questions of geographical distribution present equally 

 mterestiug problems. Take, for example, the genus Helix, 

 which ranges from the northern hmit of trees to Terra del 

 Fuego. 1200 species are described, taking the world over- 

 only 50 fossil extinct forms are known. Geographical distri- 

 bution confirms the theory of evolution, which the geological 

 record appears rather to contradict. Interest attaches to im- 

 mature forms; fixed Conchifera (as Oysters) have the young 

 free ; and naked molluscs when young are protected by a shell. 

 The MoLLuscA attain the highest degree of perfection in thei^ 

 own type ; their digestive and reproductive arrangements are 

 very perfect. As far as we know, these animals and representa- 

 tives of all its divisions existed and were as highly developed 

 throughout all geological time. 



Passing to our special subject, we are only concerned with 

 two families: — 



Gastekopoda (stomach-footed).— Aquatic, 32; terrestrial 

 74 ; total, 106 British. ' 



Conchifera (bivalves).— All aquatic ; total, 15 British spe- 

 cies ; making a total of 121 species of land and fresh-water 

 shells. Of these, 113 are common to Europe north of the 

 meridian of Bordeaux, and 115 are common to Europe south 

 of the meridian of Bordeaux. 80 of these species are fossil 

 or subfossil in the Upper Tertiaries, commencing after the 

 CoraUine Crag, and including Alluvial Deposits. 

 Turnmg to our local Comity Fauna we find— 

 13 out of 15 Aquatic Conchifera. 

 26 „ 32 ,, Gasteropods. 

 7 „ 10 Slugs. 

 28 ,, 33 Helices and allies. 

 17 ,, 31 Bulimi and Pupte. 



91 121 Total. 



The chief desiderata being in the genera Verticjo and Pupa 

 there is clearly scope for further investigation. ' 



