328 EGBERT TRACY JACKSON ON THE 



attached by a byssus as are adults, and it was due to this fact that thoy were collected 

 at the same time with the adults. 



The prodissocouch of Penia, as shown in PI. xxyn. figs. 1-3, and PI. xxvin, fig. 11, is 

 very closely similar to that characteristic of Ostrea, the umbos are directed posteriorly 

 and the histological structure is homogeneous and laminar. The succeeding' dissoconch is 

 sharply marked off from the prodissocouch by the shape of the shell and by the fact, as in 

 Ostrea, that it is composed of lime deposited in a prismatic manner in aeonchyolin matrix. 

 These two characters are features as clearly marked and as striking in the young of 

 Perna as they have been shown to be in the young of the oyster, fig. 27, p. 314. The 

 byssal notch of Perna oi-iginates in the initial stages of the dissoconch, as shown in PI. 

 xxvn, figs. 2-3, viewed from the right side. It is a fact to note in our comparative 

 studies tbat there is no indication of a byssal notch in the prodissocouch valves of Perna; 

 and the same is true of the pi-odissoconchs of Spondylus, PI. xxvii, fig. 4, Avicula, fig. 

 33, p. 330, and Pecten, PI. xxvxi, figs. 8-9, all tending to prove that the embryos lived a 

 free locomotive existence as did Ostrea up to the close of the prodissocouch pei'iod and 

 crawled, if at all, on the ventral border of the valves. As viewed from the left upper 

 side the young shell of Perna, PI. xxvii, fig. 1, is auriculate, but shows no indications 

 of the byssal notch; as in the lower right valve, PI. xxvir, figs. 2-3, the same being true 

 of yonng Pectens, PI. xxvin, figs. 9-12. This is attributed to the f;ict that it is the 

 right valve which is most affected by the presence of the byssus, for that organ in moor- 

 ing the animal to its habitat passes ovei- the edge of the right valve, as the animal lies 

 on the I'ight side. 



The degree in which a special notch exists for the extrusion of the byssus and foot 

 appears to bear a direct relation to the position of the axes of the body as compared 

 with the hinge axis of the shell (relation of axes to the shell, see p. 309). In byssated 

 Pelecypods which have the anterioi; portion of the body closely underlying the hinge 

 ai-ea, a notch is always developed. As examples, may be cited Perna, Pecten, Avicula 

 and Tridacna. On the other hand, byssated Pelecypods in which, the antero-posterior 

 axis is more nearly parallel to the hinge axis, commonly there is only a slight byssal 

 notch, or none at all. Such we find to be the case in Mytilus, Modiola, Pinna and most 

 Areas. In studying these genera it is evident that when the foot is close to the hinge 

 axis a notch in the valves greatly aids in its easy protrusion; as if no notch existed the 

 valves would have to open very widely to permit of its passage. Young Pecten, how- 

 ever, in virtue of the deep notch, pi-otrndes its foot and crawls freely, while lying on the 

 right side (see studies of Pecten) , although the valves are almost closed. Genera in which 

 the foot is far removed fi-oni the hinge line can protrude it, even if of considerable size, 

 by a comparatively slight gaping of the valves, and a permanent byssal attachment 

 seldom induces a considerable notch. Those in which the foot is far removed from the 

 hinge line mostly ci-awl on the ventral border, with the valves raised perpendiculaily, not 

 as inPecten and similar genera where crawling iseftected by dragging the shell still lying 

 on its right side, the foot being extended through a special notch in the right valve. 

 Fi-om studies of Pecten I am led to believe that a notch is a feature caused by the ex- 

 tension of the foot quite as readily as by the existence of a byssus, although either or- 

 gan would affect shell growth by its permanent existence, or activity. 



The shell of Perna in the nepiouic stage has not yet acquired adult characteristics. 



