4 
CONCHIFERA. 399 
being of the same size and shape, except in the Ostreide and 
afew others. In Ostrea, Pandora, and Lyonsia, the right valve 
is smallest; in Chamostrea and Corbula, the left; whilst the 
Chamacee follow no rule in this respect. 
The biyalyes are all more or less inequilateral, the anterior 
Dorsal Margin. 
\ Anterior 
Posterior 3 
; side. 
side, 
Ventral margin or base. 
Fig. 209. Unio pictorum, L. (original), with the right valve and mantle-lobe removed ; 
a, a, adductor muscles; p, p, pedal muscles; z, accessory pedal muscle; vu, umbo; 
1, ligament ; 6, branchial orifice ; v, analopening ; 7, foot; o, mouth ; 7%, palpi. 
being usually much shorter than the posterior side. Pectunculus 
is nearly equilateral, and in Glycimeris and Solemya the anterior 
is much longer than the posterior side. The front of the 
smaller Pectens is shown by the byssal notch; but in the large 
scallops, oysters and Spondyli, 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. 
Bivalyes are said to be close, when the valves fit accurately, 
and gaping, when they cannot be completely shut. In Gastro- 
cheena (Pl. XXIII., Fig. 15), the opening is anterior, and serves 
for the passage of the foot; in Mya it is posterior and siphonal; 
in Solen and Glycimeris both ends are open. In Bysso-arca 
(Pl. XVII., 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, s), 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 wavy; in Tellina fabula 
it is confined to the right yalye. In many species of Pholas. 
