BIVAH'E SHELLS— GEXERAL STRUCTUKE. 617 



issued from the egg can be seen crowning the umbones ; they generally differ 

 in shape and marking from the portion subsequently formed and are called 

 the prodissoconch. 



The umbo is generally curved more or less to one side, and usually points 

 towards the head, or anterior end, when the shell is said to be j)roso(jyre, in 

 contradistinction to those in which the umbones are straight, orthogyre, or 

 are directed backwards, opisthogyre. More often than not the two valves 

 are of equal size and shape, and the shell is equivalve, as in the cockle ; 

 sometimes, however, as in the oyster, one valve is smaller than the other, 

 and the shell is inequivalve. The bivalves are all more or leas inequilateral; 

 that is to say, if a line be drawn from the umbo to the ventral margin of the 

 valve the portion on one side of that line, usually the front one, will be 

 found to be smaller than that on the other. When the shell is shut it is 

 said to be dose, if the valves fit accurately ; and to gape, if openings be left. 

 The ligament is made up of two elements : an external horny, non-elastic 

 layer, or ligament proper, and an inner, fibrous layer, the resilium, which 

 imparts elasticity to the whole. The latter is sometimes separated from the 

 outer layer and situated in a pit, resilifer, in the margin of the shell, or 

 even borne on a process projecting into the interior. This resilium was 

 formerly known under the misleading name of cartilage. The margin 

 of the shell, on which the ligament and the teeth that lie close to it 

 are situated, is termed the hinge line: it is sometimes expanded inwards 

 and forms a sort of platform, the Jiinge plate, on which the teeth are 

 placed. The central teeth, under the umbo, are termed cardinal, those 

 on each side lateral teeth : frequently some one or other of these, and 

 sometimes all, are wanting. The mode of origin of these teeth is a matter of 

 extreme interest and importance, but is too special a subject to be followed 

 out here. On the interior of the shell in each valve are impressed markings 

 or scars, which indicate the points of attachment of the muscles. Firstly, 



Fig. 5.— ToPoaKAPHT op a Bitalyb ShelIi. 

 I. Exterior of rijfht valve. 11. Dorsal view of both valves. 



III. Interior of right valve. 

 A, Anterior or front. P, Posterior or hinder end. D, Dorsal area or back. V, Ventral area or 

 under margin, c. Cardinal teelh. al, Anterior lateral teeth, pi, Posterior lateral teeth. 

 aa, Scar of anterior adductor muscle, jja, Soarof pooterior adductor muscle, p, Pallial line, 

 pa, Pallial sinus. 



there are in the majority of shells— e. jr., the fresh water mussel— two large 

 impressions in each valve, one near the front, the other near the hinder, 

 margin ; these are the scars of the adductor muscles that close the shell. In 



