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 wmbo (p. 29). 
The beaks (wmbones) are near the hinge, because that side 
crows least rapidly, sometimes they are quite marginal; but 
they always tend to become wider apart with age. The beaks 
are either straight, as in Pecten; curved, as in Venus; or spiral, 
as in Isocardia and Diceras. In the latter case each valve is 
like a spiral univalye, 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 lef¢- 
handed spiral like the reversed gasteropods. When one valve 
is spiral and the other flat, as in Chama ammonia (Fig. 224), 
Mir 
Fig. 208. River-mussel. (Anodon cygneus 9) * 
the resemblance to an operculated spiral uniyalye becomes very 
striking. 
The relation of the shell to the animal may be readily deter- 
mined, in most instances, by the direction of the wmbones, 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. t 
The Conchifera are mostly equivalve, the right and left valves 
* The valves are forcibly opened and the foot (f) contracted; a, anterior adductor- 
muscle, much stretched; y, p, palpi; g, inner gills; 0, o, outer gills distended with 
spawn; 0, b, a bristle passed through one of the dorsal channels. 
+ Linneus and the naturalists of his school described the front of the shell as the 
back, the left valve as the right, and wice versa. In those works which have been 
compiled from “ original descriptions”? (instead of specimens) sometimes one end, 
sometimes the other, is called untertor; 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 
gt right angles to it. 
