64 CONTRIBUTIONS 
a long straight lateral tooth; cicatrices scarcely visible ; 
margin entire 4 
Diam. 1-20th, 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 strie 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. 
Al. minutissina. 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 1-20th, Breadth 1-20th, 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 
