46 CONTRIBUTIONS 
revoluta (Sowerby), but seems to differ in the angle of the 
posterior part. 
C. Murchisonii. Plate 1. Fig. 13. 
Description. Shell much inflated, triangular, subbian- 
gulate behind, nearly equilateral, very inequivalve ; right 
valve covered with regular, large, transverse ribs, which 
terminate at the sharp carina of the umbonial slope ; left 
valve smooth or slightly wrinkled, with two or three obscure 
ribs passing from the beak to the basal margin, and a linear 
umbonial slope, posterior to which there is a deep fold ; 
beaks very large and incurved ; tooth and pit of the left 
valve large ; posterior slope truncate, furnished with a dou- 
ble carina on the right valve and a single carina and fold 
on the left. Cicatrices not distinctly impressed ; cavity 
of the beaks very deep. 
Diam. .3, Length .5, Breadth .6, of an inch. 
Observations. 1 have peculiar pleasure in dedicating this 
curious and beautiful species to the late president of the 
Geological Society of London, who, by his numerous and 
excellent papers on geology, has eminently contributed to 
promote a knowledge of his favourite science. The Mur- 
chisonii is remarkable for the great disparity of the size of 
its valves, and their total dissimilarity. It has some re- 
semblance to the elegans (Sowerby). 
C. gibbosa. Plate 1. Fig. 14. 
. 
Description. Shell very transverse, somewhat inflated, 
nearly equilateral, very inequivalve, on the right valve 
