BIVALVE SHELLS. 225 



as being somewhat akin to the plagiostoma; as it also is to the venus, 

 the area, the mytilus, the cardium, and the tellina. Some species of 

 trigonia have been fomid in our strata ; chiefly in the oolite, and the 

 calcareous sandstone, but partly also in the dogger over the alum rock. 



Fig. 18, PI. VIII, is a specimen of the trigonia clavellata, from 

 the oolite at Pickering; where this kind of shell is principally found. 

 It is handsomely studded with tubercles, disposed in rows ; and 

 exactly agrees with Parkinson's figure, III. PI. XII. Fig. 3; and with 

 Sowerby's, Tab. 87. 



Fig. 18 a is the curious hinge of this species, as it appears in 

 the right valve. It greatly resembles the hinge of the recent shell, the 

 trigonia margaritacea, represented in Parkinson's Org. Rem. III. PI. 

 XII. Fig. 2. We have not succeeded in clearing out the hinge of a left 

 valve; but no doubt it would correspond with Fig. 1 of Parkinson's 

 Plate.* Two muscular impressions are found near the hinge; but 

 the smaller one is so close to a part of the hinge, that it is apt to be 

 confounded with it, or overlooked, especially in the right valve : yet 

 in some of our specimens, particularly in a left valve, the smaller 

 impression is very distinct. 



Fig. 19 is a beak of the trigonia costata, also from the oolite. 

 It is less common than the t. clavellata. The umbones in this shell 

 are adorned with smooth ribs, instead of rows of knobs. The t. 

 aliformis and one or two additional species are said to have been 

 found in alluvial nodules ; but we have not had an opportunity of 

 examining them. 



A beautiful trigonia occurs in our doggei-, at Falling Foss 

 and some other places, differing from any which we have hitherto 



* Either that author has by mistake interchanged the words right and left, or has 

 adopted a different definition of these terms, as applied to valves, from that of Burrow, in his 

 Elements of Conchology, p. 33. 2nd. Edit. Hence our right valve corresponds with Mr. 

 Parkinson's left valve, and vice versa. 



3 K 



