CONCHIFEEA. 



399 



being of the same size and sliapc, except in the Ostreidce and 

 a few others. In Ostrea, Pandora, and Lyonsia, the right valve 

 is smallest ; in Chamostrea and Corlula, the left ; whilst the 

 Chamacece follow no rule in this respect. 

 The bivalves are all more or less inequilateral, the anterior 



Dorsal Margin. 



Posterior 

 side. 



Anterior 



side. 



Ventral margin or base. 



Fig. 209. Vniopictorum,'L. (original), with the right valve and mantle-lobe removed ; 

 a, a, adductor muscles ; p, p, pedal muscles ; x, accessorj- pedal muscle ; ri, umbo ; 

 /, Ugament ; 5, branchial orifice ; v, anal opening ; /, foot ; o, mouth ; t, palpi. 



being usually much shorter than the posterior side. Peduncidus 

 is nearly equilateral, and in Glycimeris and Solemya the anterior 

 is much longer than the posterior side. The front of the 

 smaller Pectena is shown by the byssal notch ; but in the large 

 scallops, oysters and Sjjondyli, the only indication of the posi- 

 tion of the animal is afforded by the large internal muscular 

 impression, which is on the posterior side. The ligament is 

 sometimes between the umbones, but is never anterior to them. 

 The siphonal impression, inside the shell, is always posterior. 



Bivalves are said to be close, when the valves fit accurately, 

 and gaping, when they cannot be completely shut. In Gastro- 

 chcena (PI. XXIII., Fig. 15), the opening is anterior, and serves 

 for the passage of the foot ; in My a it is posterior and siphonal ; 

 in Solen and Glycimeris both ends are open. In Bysso-arca 

 (PI. XYII., Fig. 13), there is a ventral opening formed by 

 corresponding notches in the margin of the valves, which serves 

 for the passage of the byssus ; in Pecten, Avicula, and Anomia 

 (Fig. 211, 5), the byssal notch (or sinus) is confined to the right 

 valve. 



The surface of bivalve shells is often ornamented with ribs 

 which radiate from the umbones to the margin, or with con- 

 centric ridges, which coincide with the lines of growth. Some- 

 times the sculpturing is oblique, or wa"v^ ; in Tellina fahida 

 it L<j confined to the right valve. In many species of Pholas 



