ORGANS OF SUPPORT, 



FIG. 21. 



is termed the muscular 

 impression. When the 

 shells have a short and 

 round form, as the spon- 

 dylus, (Fig. 21. 1.) the 

 anomia, (Fig. 21. 2.) the 

 ostrea, there is commonly 

 but one adductor muscle, 

 and one muscular im- 

 pression, and these are 

 monomyaria ; when the valves have a more lengthened form, as 

 the area, (Fig. 21. 3.) the tellina, and many others, there are at 

 least two muscles and two muscular impressions, and these 

 form the dimyaria. The hinge of the valves is supported 

 by the ligament, and formed by the teeth which afford 

 the most obvious, regular and constant characters for the 

 discrimination of these shells. The teeth are seen in the 

 figure of the area barbata, (Fig. 21. 3. #,) projecting from 

 the whole extent of the hinge, and the two muscular im- 

 pressions are seen in the interior towards the ends of 

 the valves. The epidermic filaments which give a barbed 

 appearance to the surface of this and many other shells, 

 are merely uncalcified marginal projections of the albumi- 

 nous matter which pervades all the successive layers of 

 the valves. In the anomia, (Fig. 2 1 . 2, and 2*,) there is a 

 circular perforation (a,) in the flat valve, through which the 

 central portion of the adductor muscle passes to be inserted 

 into a third small circular calcareous piece, which is perma- 

 nently fixed to some extraneous solid body. The pe- 

 riphery of the adductor muscle is inserted into the flat 

 valve around the margin of the hole ; so that the muscle 

 here secretes the calcareous matter of the small fixed 

 opercular piece into which it is inserted, as the muscular 

 foot of a gasteropod secretes the calcareous operculum 

 which covers the orifice of its shell. There is a great 

 difference in the extent to which the right and left valves 

 resemble or differ from each other. Some are equivalve, 

 or have their two valves alike in form, as the cardium, 

 the mactra, the solen, the pholas ; some, as the tridacna, 

 are sub-equivalve, or have them nearly alike ; and others 

 as the anomia, the terebratula, the gryphcea, are inequivalve, 



