56 Desciiptions of Preparations. 



proposed, to the effect that the prismatic layer has been formed 

 by some such rearrang-ement of the particles deposited in the inner 

 nacreous layer, as takes place in certain calculi, after the primary 

 deposit of the solid substance of which they are made up in an 

 amorphous condition, or in the alteration of an amorphous glass into 

 the so-called '^Reaumur^s porcelain/ The '^ ligament^ which connects 

 the two valves along- the dorsal line, posteriorly to the beaks of 

 the umbones, consists of two layers — an outer corresponding to the 

 epidermal layer of the shell ; and an inner, the so-called ' cartilage,' 

 which combines by its two sets of striae, one running vertically 

 and the other horizontally, the laminate appearance characteristic 

 of ' nacre,' with the ' columnar ' characteristic of the prismatic layer 

 of shell, and illustrates thus the fundamental morphological and 

 developmental identity of these structures. The ligament in the 

 Anodon is said to be ' external ; ' it is, however, a little overlapped 

 on either side for a short way by the superficial layers of the 

 valves which it connects, and divaricates when not antagonized 

 by the adductor muscles. Corresponding to the ligament, there is 

 on the inner surface of the shell a long low ridge, indented at its 

 posterior end ; a similar structure exists in the pearl mussel, Unio 

 Margaritifer ; and seems to be a rudimentary representative of the 

 elongated posterior lateral tooth, developed in the two other British 

 Naiades, Unio Pictorum and Unio Tumidus. The cardinal teeth are 

 absent also in the Anodon, and the anterior laterals are wanting in 

 all the British Naiades. There are three principal muscular de- 

 pressions on the inner sm'face of each valve, two towards the upper 

 edge of the blunted anterior end, and one at about the same 

 distance from the dorsal or haemal edge of the valve, and about 

 midway between the posterior limit of the ligament and that of 

 the valve. Of the two anterior depressions, the larger marks the 

 place of attachment of the anterior adductor of the valves, and also 

 of the anterior retractor of the foot ; the smaller, which is close to 

 the posterior inferior aspect of the larger in the angle between it 

 and the pallial line, marks the place of attachment of the protractor 

 of the foot. The larger part of the posterior muscular depression 

 corresponds to the insertion of the posterior adductor, the irregular 

 process into which its anterior and upper angle is prolonged into, 

 corresponds to the point of origin of the posterior and larger re- 

 tractor of the foot. An irregularly denticulated line, which is 



