318 FOSSIL OSTEFID^ OF NORTH AMFRICA. 



slightest iutimation of external danger forcibly contracts, closing the 

 free edges of the valves tightly. The dark purple sears near the centers 

 of both valves, and vulgarly supposed to indicate the position of the 

 heart, are simply the areas covered by the attachment of this adductor 

 muscle, which is composed of a vast number of extremely fine muscular 

 fibers, which collectively pass straight across the space between the 

 inside of the valves, being firmly fixed at either end to the latter. 



The muscle M when closely examined is found to be composed of 

 bundles of fibers composed of still more slender fibrils, which are the 

 contractile elements of this structure in the oyster. These fibrils are 

 analogous to somewhat similar minute elements in our own muscles, to 

 which uijiscular contractions are primarily due when evoked by some 

 stimulus, such as that communicated by a very fine nerve fiber ending 

 on the surface of the muscular elements. Two kinds of fibers are also 

 found in the adductor; the first of which is the whitish, glistening variety 

 found in the hinder crescent-shaped part of the muscle, and the second 

 a paler, less lustrous, and grayish kind found in the darker and more 

 anterior portion, as has been indicated in Fig. 1, by a diflerent shading 

 of the two muscular areas. 



Another small muscle is found on either side of the body of the soft 

 parts, an inch or a little more in front of the great adductor. Its posi 

 tion and size are indicated in Fig. 1, at p', where it passes out through the 

 mantle a little vfny behind and above the palps i?, to be inserted into or 

 attached to the inside of the valves a little distance behind the hinge h. 

 This small muscle does not pass across the space between the valves 

 like the great adductor, but its inner end is soon lost in the lower an- 

 terior part of the body mass ; it is in fact a paired structure, the one on 

 the right side of the body being a repetition of that on the left. In the 

 American oyster its insertion on the inside of the valves is sometimes 

 marked by a small purplish scar about one eighth of an inch in diameter. 

 This muscle has been identified with the pedal muscle of other moUusks 

 by Dr. W. H. Dall, who was, I believe, the first to call attention to its 

 existence. 



As the oyster grows the insertions of both the great adductor and the 

 pedal muscles enlarge and are extended i)rogressively backwards on the 

 inside of both valves and away from the hinge, as may be learned upon 

 examining the muscular insertions on the valves of an oyster recently 

 opened. There are no other points of attachment between the soft 

 parts and the valves, except the opposite extremities of the great ad- 

 ductor and the small pedal muscles of either side. The soft parts are 

 therefore in life adherent to the shell at four points. 



The foregoing paragraphs fairly describe the mechanism of the shell 

 and the manner of its relation to the soft parts, and also partially indi- 

 cate the reciprocal ])hysiological relationship subsisting between both. 



The structure of the shell is laminar, or, in other words, it is composed 

 of very numerous and thin parallel layers of calcic carbonate ^chalk), 



