REPRODUCTION AND ARTIFICIAI. PROPAGATION OF FRESH-WATER MUSSELS. 147 



around the free margin. At the hinge margin two denser areas may be observed, which^ 

 when examined from the inner face of the valve, are found to be continuous with the 

 border around the free margin (fig. 13, pi. viii). The test with acid shows that this entire 

 border is calcLferous and that there is a thinner layer of carbonate of lime over the whole 

 surface and beneath the cuticle. This layer is often cracked, as one might break the 

 shell of a hen's egg, when preserved specimens are slightly crushed under a cover glass, and 

 it is then seen to be distinct from the cuticle which may wrinkle but does not break. Upon 

 the loss of the lime, the cuticle is no longer firm enough to preserve the shape of the shell and 

 successful permanent mounts must therefore avoid acids at any stage of the preparation. 



Along the ventral border of the shell is a flange, formed of cuticle only, and so 

 transparent that it is easily overlooked in a ventral view of the open glochidium (fig. 15, 

 pi. viii). Viewed laterally (fig. 14, pi. viii), this flange has at a certain focus the appear- 

 ance of a hook and may easily be mistaken for one when seen under a low magnifica- 

 tion. It is, however, a continuous flange, as shown in the figures, and not a hook ; and 

 since its edge is very fine it must, when the glochidium closes its valves, cut into and 

 hold to a delicate tissue like that of the gill filament, thus performing much the same 

 function as the hook in the other type of glochidium. The general spoon-like character 

 of the valves is shown clearly by the figures. The adductor muscle is well seen in the 

 living specimen, being a conspicuous object from whatever angle it is examined. Viewed 

 laterally (fig. 13, pi. viii), or from the ventral aspect (fig. 15, pi. viii), the adductor is 

 seen to lie nearer the shell margin at one end of the hinge than at the other, a fact which 

 enables one to recognize at a glance the future anterior border of the shell. There is 

 also in this glochidium of LampsUis subrostraia a slight difference in outline by which 

 these anterior and posterior borders of the shell may be distinguished (fig. 13, pi. via), 

 while in the hooked type of glochidium (fig. 10, pi. viii, and text fig. i, A, b, and c) 

 this difi'erence is even more pronounced and one recognizes the anterior border of the 

 future adult by its slightly greater length. 



The two outer pairs of sensory cells with their fine projections (fig. 14 and 15, 

 pi. viii) are readily seen in the living glochidium; the two inner pairs, in which the cells 

 project but a short distance from the surface, are more easily found in specimens which 

 have been properly preserved and stained. The position of the two outer pairs may 

 also be seen in the closed glochidium (fig. 13, pi. viii). Little can be seen of the rudi- 

 ments of the various organs of the adult without the careful staining of well fixed 

 material. In the living glochidium they appear as a slightly denser area on either side 

 of the median line and posteriorly to the adductor muscle (fig. 13, pi. viii). The cells 

 of the larval mantle (fig. 15, pi. viii), which occupy the greater part of the surface 

 exposed within the valves, appear in the living glochidium as a dense mass in which 

 cell outlines can not be recognized. 



Further details in the structure of this glochidium can only be studied in specimens 

 which have been properly fixed and stained. After trying various reagents, we have 

 found that they may be stupefied in a few moments by the addition of several small 

 crystals of hydrochlorate of cocaine to the water in a watch glass, after which they 



