8 G- 0. Sårs. 



by a sort of hinge, having inside an elastic ligament which 

 tends to keep the valves apart, By the aid, however, of a 

 strong adductor muscle, traversing the body and joining 

 each valve near its centre, the shell admits of being 

 perfectly closed, in which case all the parts of the animal 

 are enclosed within the cavity of the shell. Each valve is 

 composed of an outer chitinous lamella of a rather firm 

 consistency, and of a very delicate inner membranous lining 

 forming the immediate continuation of the skin of the 

 body. Between these 2 lamella? some internal organs are 

 continued, viz., the o variai tubes and the cæca of the 

 intestine (see fig. 4). 



The shell, in the living state of the animal, exhibits 

 a yellowish green colour, dotted with darker green, and 

 has moreover, dorsally, as also at each extremity, irregular 

 shadows of a much darker hue, formiug partly interrupted 

 transverse bands (see figs. 1 and 2). On each side of the shell, 

 behind the central muscular area, may generally be observed 

 a more or less distinct orange tinge, produced by the ripe 

 ova accumulated within the underlying body, and a dark 

 diagonal stripe is also seen running from the upper side 

 of the muscular area obliquely backwards. This stripe forms 

 the boundary between the ovarial tubes and the cæca of 

 the intestine, both lying, as above stated, between the two 

 lamellæ of the valves. 



When the one valve is carefully detached *),. the 

 enclosed animal can be to some extent examined in its natural 

 situation lying within the other valve (see fig. 3). It is 

 thereby easily demonstrated, that the body does not nearly fill 



*) This may b j best accomplised by previously killing the animal in 

 boiling water, when the valves will remain wide open, and 

 the several appendages extended in their natural position. 



