TO GEOLOGY. AG 
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. 
bd 
Observations. This species has some resemblance to the 
Murchisonii, 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 Corbule 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 
