TO GEOLOGY. 47 



transversely and distinctly striate, on the left valve trans 

 versely and minutely striate ; beaks elevated, incurved ; 

 umbonial slope with a double carina on the right valve, 

 on the left a single carina and indistinct fold. Cicatrices 

 not distinctly marked ; cavity of the beaks rather deep. 

 Diam. .2, Length .3, Breadth .5, of an inch. 



Observations. This species has some resemblance to the 

 J\Iurchisonii, but differs in being more transverse, in being 

 more produced behind, in having a more obtuse carina, and 

 in being less inflated. 



C. compressa. Plate 1. Fig. 15. 



Description. Shell triangular-ovate, compressed, equi 

 lateral, inequivalve, concentrically and finely striate on 

 both valves ; beaks slightly elevated, incurved ; umbonial 

 slope with a double carina on the right valve, and a single 

 one on the left ; cicatrices not distinctly marked ; cavity 

 of the beaks shallow. 



Diam. .1, Length .2, Breadth 5-20ths, of an inch. 



Observations. Not easily confounded with the preceding 

 species, being smaller and having both valves similarly 

 striate. 



The Corbulw seem to pervade most of the formations 

 from the Mountain Limestone to the Crag. Mr Sowerby 

 describes one (the nitida) from the Upper Freshwater of the 



