344 ROBERT TRACY JACKSON ON THE 



a totally diflei-ent asi)cct from that of later stages and also dillerent from any form 

 known in other species of the genus. 



The right valve of young Pecteii, I'rradiaiis, on which it rests, PI. xxviii, fig. 10, is 

 marked by a deep b^ssal notch, originating as stated in the initial stages of the disso- 

 conch shell as indicated by the lines of growth shown in the enlaiged figure of the hinge 

 area in PL xxvii, fig. 9. The border of the notch on ils lower side is marked by a well 

 defined tooth-like process, PI. xxviii, fig. 10. In later growth, PI. xxviii, fig. 13, more 

 tooth-like processes develop and they are characteristic of young specimens up to four 

 or five centimeters in height but none exist in the adult shell. The left upper valve 

 overhangs the right lower valve by a slight extent along the free margins, fig. 10, as in 

 young Osti-ea and Anomia. The color of the young shell in both valves is of a light 

 yellowish-brown. 



In the consideiation of Perna (]). 328) it is shown that when the foot lies close up 

 to the hinge area in Peiecypods a si)ecial notch exists for its extrusion. The notch is 

 I'equired because if not existent the valves would have to open very widely to allow of 

 the passage of the foot at a position so close to the i)oint of union of the valves. Per- 

 haps an equal reason for the need of the notch is the fact that such forms lie on one 

 valve and drag the shell in crawling by extending the foot over the edge of the lower 

 valve. They do not creep on the free bordeis of the valves as do ordinary dimyarians 

 in which the foot is extended from a point far removed from the hinge line by a slight 

 gaping of the valves, without the aid of any special sinus. As shown on p. 333, JPecten 

 irradians, when very young, PI. xxviii, fig. 1, has no byssal fixation, the animal leading 

 an active crawling existence. The notch may therefore be considered as a foot notch 

 rather than a byssal notch at this period. The existence of a byssus or the constant 

 extension of an active foot would equally atfect shell growth by exerting pressure on 

 the mantle boi'der, thus causing its local retraction and retai-ding shell deposition at the 

 point of extrusion, so that it is not necessary to imply the existence of a ])yssus because 

 a notch exists at an area whei'e the byssus would be produced if it existed.^ This has 

 important bearing on fossil forms Avhich are considered as byssated or attached, if a 

 notch exists in this region; whereas they may with equal i-eason be considered, as far 

 as the notch is concerned, as free forms which crawled Avhile lying on one side with the 

 foot extended through a special notch produced by the existence of such a habit. This 

 mechanical explanation of the formation of the notch in Pecten shows what close re- 

 lations may be traced between the form of hai-d parts and the anatomy and habits of 

 the animal. 



A view of the hinge area and byssal notch of a Pecten of the same age as the above 

 considered specimen viewed fi'om the right side is shown in PI. xxvii, fig. 9. The re- 

 lation of the prodissoconch to the dissoconch and byssal notch is clearly indicated and 

 it is noticeable that the left umbo jirotrudes beyond the right. This is due to the greater 

 convexity of the left valve, as is also noticeable in Chlamys island tea, PI. xxvir, fig. 8. 



The early nepionic stages of Pecten niagellanicus, P. dislocafun (ti'accd by following 



'III this connection a comparison may bo made of the PI. xxix, fig. 6. The early condition of Pecten does muph 



early nepionic sheU of Pecten, PI. xxvni, tig. 10, viewed to elucidate the somewhat similar form of shell growth of 



from the right side with a similar view of young Anomia, yonng Anomia. 



