64 CONTRIBUTIONS 



a long straight lateral tooth ; cicatrices scarcely visible ; 

 margin entire ] 

 Diam. l-20th, Length nearly .1, Breadth nearly .1, of an inch. 



The smaller figure is of the size of nature. 



Observations. This species at first sight might easily 

 be mistaken for the parva. The microscope will, however, 

 show very characteristic differences. It is less angular 

 above ; the transverse striae are much further removed ; 

 it might be said to be sulcate. All the specimens I have 

 (five) are without a crenulated margin. More perfect 

 specimens may, nevertheless, exhibit this character. 



A. minutissima. Plate 2. Fig. 39. 



Description. Shell scaleniform, acutely angular above, 

 rather compressed, oblique, ribbed longitudinally; beaks 

 elevated, pointed ; lunule very large, cordate ; cardinal 

 teeth small, the posterior margin furnished with a long 

 straight lateral one ; cicatrices scarcely visible ; margin 

 largely crenulate. 



Diam Length l-20th, Breadth l-20th, of an inch. 



The smaller figure is of the size of nature. 



Observations. Differs from the two preceding in size, 

 and may at once be distinguished by its longitudinal ribs, 

 the others being transversely striate. 



One species only seems to have been found in the beds 

 of the London Clay in England, the rugatus of Sowerby. 

 From the Lias to the Crag twenty-eight species in all have 



