36 EAMBLES IN SEARCH OF SHELLS. 



But to return to the mussels of the London waters, 

 Unio Uimidus and ly'ictorum (PI. III., figs. 1, 2) 

 are sometimes found in the same locality, and there 

 are two or three varieties of each. 



The former is a thick-shelled heavy species, oval 

 in shape, hlack in colour, and with the hinge very 

 prominent and swollen ; hence its specific name. 

 The latter is ohlong rather than oval, hroader in 

 proportion to its size, and lighter ; the shell thinner, 

 of a green or olive-brown colour, finely striated, and 

 the hinge less prominent. Both produce pearls, 

 though of a very small and inferior size, and gener- 

 ally speaking the pearly secretion takes the form of 

 an irregular mass deposited on the shell. The use 

 which painters have found for the shells, viz., to hold 

 their colours, has evidently suggested the specific 

 name pictonim. Besides the London waters, in- 

 cluding that in Battersea Park, the ponds at Hamp- 

 stead and Highgate, the reservoirs at Kingsbury and 

 Elstree, the river Brent between Hendon and Brent- 

 ford, to say nothing of the Thames itself, are localities 

 which may be named to such as are desirous of 

 obtaining mussels ; while those who delight in the 

 exercise of a long country walk may give attention if 



