1 74 iirSCELLAXEA. 



upwards conformably and regularly into the undisputed lime- 

 Btone series. Associated with these shells in Ireland are the 

 fronds of a magnificent feni Cyclopteris Hihernicus, Forbes, and 

 several other species of plants, some of which pass up into higher 

 and better-known, Carboniferous beds. 



The bed of sandstone in which tliis shell has been found forms 

 a very thick stratum in the JS'orth of Northumberland, and is a 

 striking physical featui'e on the east side of the Till for many 

 miles. It is a fine grained sandstone, hard and suitable for build- 

 ing pui'poses in many places, with patches of very friable nature 

 between the hard layers filled with reddish, irregular nodules of 

 clay. 



Eemains of Ulodendron ornatissimiim, branches of a Lepidoden- 

 droid plant, and stems of a Calamite not yet identified occur in 

 the same stratum. This sandstone bed is below all the beds of 

 Carboniferous limestone, and forms probably an upper member 

 of the Tweedian gi-oup of the late Mr. George Tate, a group of 

 strata closely identical with the lower portion of the Calciferous 

 sandstones of Scotland. The supposed Ujiper Old-lied Beds of 

 Kilkenny will also no doubt be eventually and correctly coire- 

 lated with these. 



On Plate XIV., a figure, a, is given of the largest specimen 

 about the natural size. The length when perfect would be about 

 nine inches, and the breadth three inches and one quarter. The 

 anterior extremity is imperiect. The dorsal margin arcuated 

 with slight indications of the umbones, before which there seems 

 to have been a slight thickening of the sliell. The hinge-margin 

 is slightly arched. Tlie posterior margin slopes downward, and 

 the ventral is slightly curs'ed upwards under the umbones. The 

 general form resembles the recent Vnio margariiifera, Linn., of 

 our northern streams. The hinge-line shows no trace of teeth 

 and was, as far as tlie presentation of these specimens shows, 

 quite straight. The shell appears to have been veiy thin, as in- 

 dicated by fine concentric undulations on tlie surface of the cast. 

 There are no traces of muscular impressions. The smaller spe- 

 eimcD, fig. h, is more unioniform in shape, which has partly arisen 

 from tlie circumstanoe that it was resting in a sloping direction 



