290 POPULAR BRITISH CONCHOLOGY. 



dorsal margin^ and the shell is then described as nearly 

 equilateral ; sometimes they are near one end^ and then the 

 shell is very inequilateral. In the Mussel {Mytllus) they 

 are terminal, no part of the anterior end of the shell pro- 

 jecting beyond them. They vary much in the degree in 

 which they are curved, generally only leaning towards one 

 side ; in Pectunculiis showing no perceptible bias ; in Chama, 

 coiled round in a kind of flattened spire on the back of the 

 valve ; in the beautiful Isocardia, regularly and freely spiral ; 

 and in the fossil Liceras, irregularly twisted, large, and dis- 

 tant from each other. 



THE LUNULE AND DORSxiL AREA. 



On the dorsal margin of bivalves is sometimes seen a por- 

 tion, or area, separated from the rest, either by a line or by 

 a difference of texture, or by being hollow or depressed. 

 When this is on the posterior side of the umboes, it is 

 termed the dorsal area ; when on the anterior side, it is the 

 lumde. When looking upon the dorsal edge, with the front 

 towards the spectator and the valves closed, the lunule ap- 

 pears heart-shaped or round ; and sometimes it forms a kind 

 of button, encircled by a deep groove ; in other cases it is 

 flat, and not distinctly circumscribed. 



