342 ROBERT TRACY JACKSON ON THE 



apparently direct currents of water in channels as it flows through the grate-like gill. 

 When the i-eflected border of the filament ceases to cover the dii'ect border, this Avail is 

 built into a peculiar plicated ridge or blade, hi (fig. cited), from the direct border of the 

 filament, and continues in this blade-like fashion to the point of origin of the filament. 

 The gills are suspended by fasciated membranes which connect the bases of the two 

 pairs with the adductor muscle, with the visceral mass anteriorly, and with the mantle 

 lobes posteriorly. Similar fasciated membranes also exist in Anomia ylahra, PI. xxix, 

 figs. 1-2. 



X. Pecten. The Shell. 



Characters are found in the young developing shell of Pecten irradians and other 

 species which are widely difierent from those peculiar to the adults and a consideration 

 of these features will now be taken up. 



The first specimens of young Pecten irradians obtained were found in the drain-pipe 

 ti'aps at Buzzards Bay on the fourth of August. The youngest of these specimens is 

 shown in PI. xxviii, fig. 9, viewed from the left ujiper side, while fig. 10 is the same 

 individual viewed from the right side on which the animal habitually lies. PI. xxvir, fig. 

 9, is a similar specimen viewed from the uml^onal area of the right side, more enlai-ged. 

 The prodissoeonch, j), hi the several figures is as sharply marked ofi" from the succeed- 

 iug dissoconch as in Ostrea, Perna and Avicula, Pis. xxiv, xxv and xxvir. It is sep- 

 arated from the dissoconch as in the genei'a mentioned, by its form, histological sti'uct- 

 lu'e and inferi-ed anatomical features. The umbos of the prodissoeonch of Pecten ai'e 

 directed posteriorly; but they are not as pronounced and arcuate as in Ostrea and Perna. 

 The structure of the prodissoeonch is homogeneous and laminar, showing fine concentric 

 lines of growth. There is not the slightest indication of the byssal notch in the prodis- 

 soeonch valves; but it originates in the initial stages of growth of the succeeding disso- 

 conch, PI. xxvir, fig. 9 and PI. xxviii, figs. 10 and 13. As was argued when discussing 

 Perna and Avicula (p. 329), the position of the byssal sinus in its relation to the pro- 

 dissoeonch shell indicates that the foot of that period of development occupied a position 

 on the free ventral borders of the valves, its normal position in dimyarian Pelecypods. 

 The prodissoeonch stage of Pecten was doubtless dimyarian, a supposition confirmed by 

 the shape of the shell, the position of the foot and the aflinities of Pecten to Avicula 

 and Ostrea, in which last genus two adductor muscles are shown to exist at that stage, 

 PI. xxiv, figs. 1-2. 



The habits of the prodissoeonch stage of Pecten, we know only from analogy and 

 the structui'e indicated by the hard parts; but as it differs fi'om the Ostrean prodisso- 

 eonch in possessing a foot, it may be at this period, a veliger swimmer as is Ostrea or 

 a purely crawling form. If the latter, it doubtless crawls on the venti'al border of the 

 valves as is the habit in Pelecypods in which the foot is extended from between the ven- 

 tral border of the valves opposite the hinge line. Only very young Pectens show the 

 prodissoeonch still intact, as it is not retained in this genus as long as in the sedentary 

 oysters, Aviculas and Pernas. The prodissoeonch of Pecten as in Ostrea, Perna, Avic- 

 ula, etc., is ti'acenble in origin to the N^iiculoid radical, from which the Avieulidaj and 

 their allies were (U'rived (discussion, section xv, and table, section xvr). 



