BIVALVE SHELLS. 5*7 



valve is left, and vice versa. Sometimes also there are posterior 

 lateral teeth or laminae of the hinge-margin which are found on 

 the right or left side, according as the valve is right or left. 

 When these distinguishing marks of the interior are absent, the 

 valves can be more readily determined externall}^ 



In nionomyary and attached shells it is sometimes difficult to 

 distinguish the valves. The single muscular scar is on the pos- 

 terior side of the interior, and in Pecten, the byssal sinus is 

 below the ear on the anterior side (ii, 32). 



Linngeus and the naturalists of his school described the front 

 of the shell as the back, the left valve as the right, and vice 

 versa. In those works which have been compiled from " original 

 descriptions " (instead of specimens) sometimes one end, some- 

 times 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. 



The conchifera are mostl}^ eqnivalve, the right and left valves 

 being of the same size and shape, except in the Ostreidse and a 

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

 smallest ; in Chamostrea and Corbula, the left ; whilst the 

 Chamaceffi follow no rule in this respect. 



The bivalves are all more or less inequilateral ., the anterior 

 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 ligament is some- 

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



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

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

 chtena the opening is anterior, and serves for the passage of the 

 foot ; in Mya it is posterior and siphonal ; in Solen and Glyci- 

 meris both ends ai'e open. In Byssoarca 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 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 concen- 

 tric ridges, which coincide with the lines of growth. Sometimes 

 the sculpturing is oblique, or wavy; in Tellina fabula it is con- 

 fined to the right valve. In many species of Pholas, Teredo, 

 and Cardium, the surface is divided into two areas by a trans- 

 verse furrow, or by a change in the direction of the ribs. The 

 lunule (ii, 31), is an oval space in front of the beaks; it is deeply 

 impressed in Cardium retusum, L., A.starte excavata^ and the 

 genus Opis. When a similar impression exists behind the beaks, 

 it is termed the ei<cutcheon. 



The ligament of the Conchifera forms a substitute for the 



