56 BIVALVE SHELLS. 



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 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 gastropods. When one valve is spiral and the 

 other flat, as in Chama, the resemblance to an operculated spiral 

 univalve iDecomes very striking. 



In order to properly understand the relation of the bivalve 

 shell to its owner, and thus obtain a clearer idea of the nomen- 

 clature employed in measurements, we should suppose a living 

 lamellibranchiate to be creeping upon the sea-bed, its line of 

 march directed from us. The shell will then be elevated, the 

 ligament or hinge-margin uppermost, the opposite margin 

 whence the foot protrudes lowest ; these are then termed the 

 dorsal and ventral margins respectively, and the distance 

 between them is the height of the shell. I use dorsal margin 

 in preference to beaks as a point of measurement, because in 

 some mollusks, such as Mytilus, the beaks are at the anterior 

 end : it is difficult to so locate the measurements as to suit all 

 forms of shells. The hinge-line then indicates the direction of 

 the length of the shell, and the actual length is the distance 

 between the anterior and the posterior (or collectivelj^ lateral) 

 margins. The breadth is a diameter transverse to the length, and 

 includes the distance between the greatest exterior convexity 

 of each valve. The thickness refers to the distance included 

 between the internal and the external face of a valve. When the 

 animal travels from us in this manner, hinge-line uppermost, the 

 beaks are usually nearer to, and their apices directed forwards 

 towards the anterior end, or the direction of march ; whilst the 

 ligament binding the valves together is posterior to the beaks, 

 or nearest the observer. The valves lying on the right and left 

 sides of the hinge-margin, when the shell is thus placed, are 

 respectively termed right valve and left valve. 



As the shell lays before us in the study, the valves may be 

 thus distinguished ; place the dorsal margin from you, the 

 ventral margin towards you, the exterior surface exposed ; if 

 the ligament is on the left side, you have the right valve, and if 

 on the right side, the left valve. There is frequently a lunule (ii, 

 31), or small semicircular impression anterior to the beaks ; when 

 this appears on the right side, the valve is right, and vice versa. 

 If the interior of the valve is placed uppermost, the dorsal 

 margin from us, the ventral margin towards us, we look first 

 for a sinus or flexure (ii, 31) of the pallial impression, a mark 

 made by the retractor of the siphons; if on the left side, the 



