398 



MANUAL OF THE MOLLUSCA. 



less to one side ; the apex is the point from which the growth 

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

 The beaks {umbones) are near the hinge, because that side 

 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 Peden ; 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 Concholepas ; it is the left valve which 

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

 lianded spiral like the reversed gasteropods. When one valve 

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



Fig. 208. Eiver- mussel. (Anodoji cygneus Q) * 



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 of a bivalve 

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

 from the dorsal margin to the base, and its thickness from the 

 centres of the closed valves, f 



The Conchifera are mostly equivalve, the right and left valves 



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

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

 spawn ; b,b, a bristle passed tlirough one of the dorsal channels. 



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

 back, the left valve as the right, and vice versa. In those works which have been 

 compiled from "original description's" (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. 



