398 



MANUAL OF THE MOLLrsCA. 



less to one side ; the apex is tlie point from wliicK the growth 

 of the valve commences, and is termed the beak, or umbo (p. 29). 

 The beaks {umhones) are near the hinge, because that sido 

 grows least rapidly, sometimes they are quite marginal ; but 

 they always tend to become wider apart with age. The beaks 

 are either straight, as in Pedcn ; curved, as in Venus ; or spiral, 

 as in Isocardia and Diceras, In the latter case each valve is 

 like a spiral univalve, especially those with a large aperture 

 and small spire, such as Concholejjas ; it is the left valve which 

 resembles the ordinary univalve, the right valve being a left- 

 handed spiral like the reversed gasteropoda. When one valve 

 is spiral and the other flat, as in Cliama ammonia (Fig. 224), 



Fig. 208. River- mussel. {Anodon cygneus 9 ) * 



the resemblance to an operculated spiral univalve becomes very 

 striking, 



The relation of the shell to the animal may be readily deter- 

 mined, in most instances, by the direction of the umbones, and 

 the position of the ligament. The umbones are turned towards 

 the front, and the ligament is posterior ; both are situated on 

 the back, or dorsal side of the shell. The length oi a bivalve 

 is measured from the anterior to the posterior side, its breadth 

 from the dorsal margin to the base, and its thichie^ from the 

 centres of the closed valves, f 



The Conchifera are mostly equivalve, the right and left valves 



* The valves are forcibly opened and the foot (/) contracted; a, anterior adductor- 

 muscle, much stretched ; p, p, palpi ; g, inner gills ; o, o, outer gills distended with 

 Bpawn ; b,b, & bristle passed tln-oiir;h one of the dorsal channels. 



t Linnaeus and the naturalists of his school described the front of the shell aa the 

 back, the left valve as the right, and vice versa. In those works v:hu:h have been 

 compiled from "original descriptions" (instead of specimens) sometimes one end, 

 Bometimes the other, is called anterior ; and the length of the shell is sometimes 

 estimated in the direction of the length of the animal, but just as frequently in a line 

 at right angles to it. 



