342 ROBERT TRACY JACKSON ON THE 



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

 When the reflected border of the filament ceases to cover the direct border, this wall is 

 built into a peculiar plicated ridge or blade, M (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 

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

 lobes postei'iorly. Similar fasciated membranes also exist in Anomia glabra, PI. xxrx, 

 figs. 1-2. 



X. Pecten. The Shell. 



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

 species which are widely diflferent 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 

 traps at Buzzards Bay on the fourth of August. The youngest of these specimens is 

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

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

 9, is a similar specimen viewed from the nmbonal area of the i-ight side, more enlarged. 

 The prodissoconch, ji-, in the several figures is as sharply marked ofi" from the succeed- 

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

 arated from the dissoconch as in the genera mentioned, by its form, histological struct- 

 lU'e and infei-red anatomical features. The iimbos of the prodissoconch of Pecten are 

 directed postei'iorly; but they are not as pronounced and arcuate as in Ostrea and Perna. 

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

 lines of gi'owth. There is not the slightest indication of the byssal notch in the prodis- 

 soconch valves; but it originates in the initial stages of gi-owth 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- 

 dissoconch shell indicates that the foot of that jjeriod of development occupied a position 

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

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

 the shape of the shell, the position of the foot and the afiinities 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 prodissoconch stage of Pecten, we know only from analogy and 

 the structure indicated by the hard parts; but as it differs from the Ostrean ])rodisso- 

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

 a purely ci'awling form. If the latter, it doubtless crawls on the ventral 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 

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

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

 ula, etc., is traceable in origin to the IN^uculoid radical, from which the Aviculidse and 

 their allies were derived (discussion, section xv, and table, section xvi). 



