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PROCEEDING OE THE GEOLOGICAL SOCIETY. 



The Bivalvia of the Caradoc exceed in number those of any other 

 known formation below the Carboniferous Limestone. Careful analysis 

 shows that the Arenig rocks contain only 6 species, the Llandeilo 6, 

 the Caradoc 76, Lower Llandovery 3, Upper Llandovery 29, Wen- 

 lock 45, and the Ludlow 71 species ; about 13 species range from 

 the Caradoc upwards or through to the Ludlow, 7 to the Upper 

 Llandovery, 3 to the "Woolhope, 11 to the "Wenlock, and 12 to the 

 Ludlow — this of course being inclusive, as some of the same forms 

 appear more than once in their range. It is not improbable that 

 errors as to species occur, especially when we have to do in many 

 instances with mere casts in these arenaceous deposits. In both the 

 Lower and CJpper Llandovery groups the fossils are badly preserved, 

 being casts only. If the Lamellibranchs are largely represented in 

 the Bala group, the Gasteropoda are almost equally so, by 14 genera 

 and 53 species ; 2 only (MurcJiisonia simplex, M'Coy, and Turbo 

 tritorquatus, M'Coy) are common to the Llandeilo below; 10 pass to 

 the Lower Llandovery ; 9 direct to the Upper Llandovery ; 7 species 

 are common to the Caradoc, Lower Llandovery, and Upper Llan- 

 dovery ; they are Cyclonema crebristria, M'Coy, Holop>ella cancellata, 

 Sow., H. tenuicincta, M'Coy, MurcJiisonia cancellata, M'Coy, M. 

 pulchra, M'Coy, Trochonema triporcatum, M'Coy, and Bhaphistoma 

 lenticulare, Sow. Only 3 species pass to the Wenlock rocks : one 

 of these is Holopella cancellata, Sow. The Gasteropod fauna there- 

 fore is as significant as the Lamellibranchs, no fewer than 32 of 

 the 53 species being confined to the Caradoc or Bala group. 16 of 

 the 53 are peculiarly Irish forms, and 5 Scotch, leaving 21 for dis- 

 tribution through the North- and South- Welsh and English beds, of 

 which 9 occur in the Caradoc of Shropshire. I have no determined 

 species from Westmoreland ; North Wales yields 26 species, South 

 Wales only 3, viz. Cyclonema crebristria, Holopella cancellata, and 

 Patella saturni, and these only in Caermarthenshire. This group, 

 like the bivalves, indicates shallow-water conditions. 



The Pteropoda (4 genera and 12 species) are, with 4 exceptions, 

 confined to the Caradoc. Conularia Sowerbyi passes to the Lower 

 Llandovery, Wenlock, and Ludlow ; Ecculiomplialus scoticus, M'Coy, 

 is Llandeilo and Upper Llandovery ; 9 of the known species are 

 Irish, and 3 of them Scotch ; so that the Lower Llandovery is only 

 directly allied to the Caradoc by one form (C. Sowerbyi) out of 

 the 12 occurring. This is even more strongly manifested by the 

 associated* class Heteropoda, of which, as in the Pteropoda, only 1 

 species in 15 passes to the Lower Llandovery — Bellerophon carinatus, 

 Sow., being, so far as I know, the only form in this group connecting 

 the two formations. Bellerophon bilobatus, Sow., B.perturbatus, Sow., 

 and Maclurea macromphala, M'Coy, are also Llandeilo. The Upper 

 Llandovery has 3 in common, Woolhope 4, Wenlock 2; and 2 

 pass to the Ludlow. Specifically, 6 pass up, leaving 8 as be- 

 longing to the horizon. The Pteropoda, being strictly or essen- 

 tially pelagic, give us little clue as to bathymetric conditions at 

 the time of deposition. Neither do the Cephalopoda ; but no single 

 class is so preeminently Caradoc : of the 47 known species, only 1 



