ANNIVERSARY ADDRESS OF THE PRESIDENT. 



139 



geographical distribution is significant, but clearly shows the im- 

 perfection in collecting, and how much has yet to be done to 

 demonstrate their relation to the extensive areas over which they 

 ara distributed : — 



Caernarvonshire has yielded 6 genera and 16 species. 

 Denbighshire „ 4 „ 7 „ 



Montgomeryshire „ 8 ,, 11 „ 



Merionethshire „ 6 „ 19 „ 



Pembrokeshire 



ii 



none 



none 





Caermarthenshire 



3> 



2 „ 



2 





Shropshire 



35 





10 





Westmoreland 



?> 





3 



55 



Scotland 



5J 



1 



1 



J5 



Ireland 



>> 



10 „ 



37 



55 



Nothing short of strict analysis could impress upon us the fact 

 of such unequal distribution as the above examination shows. No 

 species of bivalve is known in Pembrokeshire, a district rich in 

 other groups. Only 1 genus and species in Scotland {Pleurorliyn- 

 ehus dvpterus, Salt.), 2 only in Caermarthenshire (Ami onychia triton 

 and Ctenodonta varicosa, Salt.), and 3 in Westmoreland (Cteaodonta 

 anglica, D'Orb., Pterincea tenui striata, M'Coy, and Oardiola inter- 

 rwpta, Sow.). The physical geography of the area, either through 

 barren or interrupted coast-line, rather than movements of land or 

 depth of sea, would most probably account for this unequal distri- 

 bution and poverty of species in one area, and their comparative 

 abundance or fair representation in another. Looking at the horizons 

 which the above 6 genera illustrate, and through which they pass, 

 it seems hardly explicable that there should be only 3 species of 

 Lamellibranchiata known in the Lower Llandovery, viz. Pterincea 

 retroflexa, Mytilus mytilimeris, and Orthonota sulcata ; and these are 

 Caradoc and Upper Llandovery also, showing us that not a single 

 species belongs or is confined to the Lower Llandovery. This 

 result, compared with the large bivalve fauna in the Caradoc or 

 Lower Bala beds (57 species), and under conformable stratification, 

 is scarcely to be accounted for, except through elevation of the sea- 

 bed and change in bathymetrical conditions sufficient, with all 

 other conditions, to cause the extinction or removal to another area 

 of the Caradoc Lamellibranchs. Similar results are obtained with 

 the Gasteropoda : the 53 Caradoc and Bala forms dwindle down to 

 16 in the Lower Llandovery. The Llandovery species will be noticed 

 in their place or order subsequently. As before, Ireland yields the 

 largest number of species (37). 



Gasteropoda. — Like the three preceding classes, the group 

 Gasteropoda is largely represented in the Caradoc strata ; 14 genera 

 and 53 species have been determined. No species has yet occurred 

 in Pembrokeshire, and only 3 in Caermarthenshire {HolopeTla can- 

 ceUata, Sow., Patella satumi, Goldf., and Cyclonema crebristria, 

 M'Coy). Neither have we evidence of any species from Westmore- 



