328 EGBERT TRACY JACKSON ON THE 



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

 at the same time with the adults. 



The pi-odissoconch of Perna, as shown in PI. xxvn, figs. 1-3, and PI. xxvih, fig. 11, is 

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

 and the histological structure is hon\ogeneous and laminar. The succeeding dissoconch is 

 sharply marked off from the prodissoconch 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 aconchyoliu 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 originates in the initial stages of the dissoconch, as shown in PI. 

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

 studies that there is no indication of a byssal notch in the jirodissoconch valves of Perna; 

 and the same is true of the ])i-odissoconchs of Spondylns, PI. xxvii, fig. 4, Avicula, fig. 

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

 free locomotive existence as did Ostrea up to the close of the prodissoconch period and 

 crawled, if at all, on the ventral boi'der 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 i-ight valve, PI. xxvii, figs. 2-3, the same being true 

 of young Pectens, PI. xxviii, figs. 9-12. This is attributed to the fact that it is the 

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

 ing the animal to its habitat passes over 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 extriision 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 anterior portion of the body closely underlying the hinge 

 area, 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 JVEytilus, 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, protrudes 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 from 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 tlie foot is far removed fi'om the 

 hinge line mostly crawl on the ventral border, with the valves raised perpendicularly, not 

 as in Pecten and similar genera where crawling iseftected by dragging the shell still l^'ing 

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

 From studies of Pecten I am led to believe that a notch is a fe-.iture 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 nepionic stage has not yet acquired adult characteristics. 



