246 



MANUAL OF THE MOLLUSCA. 



The relation of the shell to the animal may be readily determined, in 

 most instances, by the direction of the umbones, and the position of the liga- 

 ment. The umbones are turned towards the front, and the ligament is pos- 

 terior; 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.* 



Dorsal margin. 



Ventral margin, or base. 



Fig. 172. Unio pictorum, L. (original) with the right valve and mantle-lobe re- 

 moved; a, a, adductor muscles ; p. p, pedal muscles; x, accessory pedal muscle; u, 

 umbo; I, ligament ; b, branchial orifice; v, anal opening; /, foot; o, mouth; t, palpi. 



The Conchifera are mostly equivalve, the right and left valves being of 

 the same size and shape, except in the Qstreida and a few others. In Ostrea, 

 Pandora and Lyonsia the right valve is smallest; in Chamostrea and Cor- 

 bula, the left ; whilst the Chamacea 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 front of the smaller Pectens is shewn by the byssal notch ; but in 

 the large scallops, oysters and Spondyli, the only indication of the position 

 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 



* Linnaeus 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, 

 sometimes 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. 



