308 



MAinJAL OP THE MOLLUSCA. 



less to one side ; the apex is the point from -whicli the growth 

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

 I'he beaks {umhones) are near the hinge, because that side 

 grows least rapidly, sometimes they are finite marginal ; bnt 

 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 left- 

 handed spiral like the reversed gasteroiDods. When one valve 

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



Fig. 20S. River-mussel. {Anodon cygneiis 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 umhones, 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. breadth 

 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 



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

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

 spawn ; b, b, & bristle passed through 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. lu those works which have been 

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

 sometimes 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. 



1 



