ANNIVERSARY ADDRESS OE THE PRESIDENT. 



149 



small bivalve fauna. Two of the three species of Conchifera are 

 Caradoc, and the same two passed to the Upper Llandovery, so that 

 in reality only 1 species is to be regarded as Lower Llandovery. 



Gasteropoda. — Although this class includes a much greater number 

 of species than the Lamellibranchiata, yet the value of the fauna for 

 classificatory purposes is equally unsatisfactory. 9 genera and 13 

 species are known ; but of these 6 genera and 10 species are Caradoc 

 and Bala forms, and 6 genera and 8 species are common to the two 

 Llandovery series. Only 1 species is known in North Wales (Mur- 

 cliisonia jpulchra, M'Coy), 5 genera and 9 species in South Wales, 

 and 6 genera with 6 species in Scotland and with 7 in Ireland. Thus 

 only 6 species of Gasteropoda are truly of Lower Llandovery age — 

 Murchisonia Prycece, M. sulcata, Platyscliisma Williamsoni, Acroculia 

 hcdiotis, Trochus Moorei, and T. multitorquatus. 



Pteropoda. — The only Pteropod occurring in the Lower Llando- 

 very is Conularia Sowerbyi, Def., and this has only appeared in South 

 Wales and Scotland ; none are known in North Wales or Ireland. 

 This species bridges over the Upper Llandovery, Denbighshire Grits, 

 and Woolhope series, and reappears in the Wenlock Shales and 

 Limestones in South Wales and Scotland. 



Heteropoda. — Bellerojphon carinatus, B. bilobaius, and B. dilata- 

 tus? are the chief Lower Llandovery species. The same 3 are Caradoc, 

 and B. carinatus and B. bilobatus also occur in the Upper Llando- 

 very of South Wales and Scotland, B. dilatatus being Irish also. 



The species of these two nocturnal and pelagic groups, the Hetero- 

 poda and Pteropoda, from their habit, had a wide geographical dis- 

 tribution, and consequently a long range in time also. 



Cephalopoda. — We have seen that the Lower Caradoc and Bala 

 rocks have yielded no less than 8 genera and 47 species ; the Lower 

 Llandovery, the highest member of the Bala formation, has, on the 

 contrary, only 3 genera and 8 species distributed through the 6 areas 

 in which this formation is represented. The genera are Lituites, 

 Tretoceras, and Orthoceras. The first-named genus has 2 species, 

 L. cornu-arietis and L. undosus ; the second (Tretoceras) 1 species, 

 T. bisiphonatum ; and Orthoceras contains 5 species, 0. annulatum f 

 0. ibex, 0. politum, 0. tenuistriatum, and 0.vagans(?). Of these no 

 species occurs in North Wales. South Wales, however, in the Llan- 

 dovery area, has yielded 5 species, Scotland 2 (Orthoceras politum 

 and 0. tenuistriatum), and Ireland 1 species (Orthoceras ibex). Pour 

 of the 8 pass to the Upper Llandovery, and 7 came from the Caradoc. 

 Tretoceras bisiphonatum and Lituites undosus are the only restricted 

 Lower Llandovery species out of the 8 known. 



The accompanying table shows the complete census of the Lower 

 Llandovery rocks, and their relationship to the Caradoc below 

 and the Upper Llandovery above, and at once reveals the fact that 

 this upper member of the Bala group has no claim to separation 

 from it ; for 50 genera and 105 species unite it with the Caradoc 

 and Bala proper, and 45 genera and 104 species pass to the 

 Upper Llandovery ; and we shall find, on casting the life-history of 

 the Upper Llandovery also, that nearly one half of both genera 



vol. xxxvn. m 



