224 HALF AN HOUE WITH SHELL-FISH (BIVALVES). 



Crenella, on the other hand, is a bivalve only half 

 an inch in length, of a pale green colour. There 

 are several British species, of which one, called 

 discors, is covered with delicate lines, radiating from 

 the beak, or umbone, to the margin. The interior 

 edges of this margin are " milled " like a shilling 

 or rather, crenulated, as the technical word has it. 

 It is this feature which gives the name to the genus, 

 although, as we have already seen, it is shared by 

 other bivalves. Crenella may be found entangled in 

 the roots of the large seaweeds (Laminaria) which 

 are cast ashore. It forms a curious nest or cocoon, 

 spun out of its byssus, and having stones, &c., 

 adhering to it and hiding it. The Lima resembles 

 it in this queer habit, although it differs much from 

 it in appearance. It has a surface cut like a file, 

 and is a very fragile and elegant shell. The valves 

 gape, and do not come quite together when shut. 



Two other British bivalves, Nucula and Leda, 

 may be known by the structure of their hinge-teeth, 

 which are very numerous and pretty. In the 

 former, there is a nacreous or pearly lining to the 

 interior of the shell, which is only about half an 

 inch in length. Leda is longer and more elegantly 

 shaped. A series of shells not likely to be mistaken 

 are the " Arks " (Area), so called onac count of their 

 chest-like form. One species, tetragona, is deep and 

 squarish. The exterior is furrowed and ridged. 

 There are several British species, which may be 

 found at low water under stones, or hiding up in the 



