24 The Irish Naturalist. February, 



usually brought up by the dredge where the depth of water 

 is five to ten fathoms. While I speak of the change as 

 rather abrupt I do not mean to say that it was sudden or 

 instantaneous, but just to point out that the stability that 

 marked the preceding period had ceased and given place 

 to a period of active depression. The maximum depth to 

 which the land subsided was not reached all at once, for 

 we find the new order of things inaugurated by the 

 appearance of the two boring shells, Pholas Candida and 

 Pholas crispata, in clay just slightly higher than the surface 

 of littoral shells. These Pholads live still in the bay at 

 low water and slightly above it, and their appearance in the 

 clay is the first intimation of the subsidence then commenced. 

 As we rise upwards in the deposit, the shells indicate a 

 greater depth of water on the bottom which they inhabited. 

 Lucinopsis undata, Cardimn echinatum, and Scrobicularia 

 alba become the most abundant fossils, and are found in 

 the utmost profusion. The range of these shells in our bay 

 is for the first-named 5-10 fathoms, for the second TO-20, 

 and for the third 8-10 fathoms. T have preferred, when 

 giving the range of these shells, to quote the reports of the 

 Belfast Dredging Committee of the British Association. ^ 

 Mr, Jeffreys gives a greater vertical range for most species, 

 but as his is the range in depth for the whole of Britain, 

 it is evident that it does not give the limits of bathymetrical 

 distribution for any one locality as accurately as does a 

 report founded on observations made in that locality. 

 Thracia convexa, Thracia papyracea, and Panopea plicata 

 are interesting shells that now make their appearance in 

 some plenty. The last-named is a rare British shell, and 

 has not been found elsewhere in Ireland. Fine specimens 

 of Axiniis flexuosus occur plentifully, and also immense 

 shells of the solitary deep-water^variety_of the Oyster — 

 Ostrea hippopus. In investigating this deposit, and making 

 a collection of its fossils, I had a twofold object before me : 

 firstly, to ascertain with precision the fauna of the period, 

 and secondly, from that fauna to deduce the climatal and 

 bathymetrical conditions under which the bed was laid 

 down. With regard to climate the result seems to be that 



1 Brit. Assoc. Reports for 1857, 1858, 1859. 



