3G0 ROBERT TRACY JACKSON ON THE 



and is less highly modified. The initial stages of dissoconch growth commence on the 

 free border of the prodissooonch valve; but succeeding shell deposition is extended pos- 

 ei-iorly and then ventrally more and more, as may be seen by folloAving the lines of 

 gi'owth, figs. 6-7, until finally a deep almost enclosed sinns is formed ai-onnd the byssns, 

 as shown in the figure. At this stage it is desiraljle to compai'e the right valve of Ano- 

 mia, fig. 6, with my figure of the first stages of dissoconch growth of Pecten, PI. xxviir, 

 fig. 10. In both, a deep and somewhat comparable notch is formed by the existence in 

 one case of a byssns and in the other of a highly active foot; both organs affecting the 

 form of the valve by causing a local retraction of the mantle and thus putting an obstacle 

 in the way of shell deposition at that area. We see in the comparisons of these figures 

 the verification of the statement made by Forbes and Ilanley nearly fifty years ago that 

 "the perforation in one of the valves of Anomia is chiefly a greater extension of the 

 auricular sinus in Pecten." In later growth the ventral border of the valve surrounds 

 the byssal notch in an increasing degree until soon it is completely enclosed. The lower 

 valve is peculiar in that the mantle border of the mnbonal region deposits shell around 

 the prodissoconch, enclosing it by degrees and partially covei'ing its umbonal portion 

 as shown in PI. xxix, figs. 6-7. This is due to the fact that the primitive umbos recede 

 from the shell border, being pushed back by the peripheral growth of the valve as seen 

 viewed from the left side in PI. xxlx, figs. 4, 5 and 8. When the byssus in young An- 

 omia is neai'ly or quite surrounded by the growth of the right valve as in PI. xxtx, fig. 2, 

 the foramen is veiy small. In the adult, fig. 41, p. 354, on the other hand, the foramen 

 of the I'ight valve is many times larger and to accomplish this increased size of the hole 

 an extensive i-esorption of the shell has taken place. During this resorption, lines of 

 growth which were continuous when young become separated and may be traced on 

 either side of the foramen, as in the figure cited. 



The lower i'ight valve in the young does not equal the size of left upper valve which 

 overhangs its border by a considerable amount, PI. xxix, figs. 6 and 2; it is also of ex- 

 cessive thinness and so delicate that great care is required in handling not to injure it. 

 The right valve shows the most beautiful, prismatic structure of which it is entii'cly com- 

 posed in early stages, and it is so transparent that the organs can be seen as through 

 a glass fihn. The extreme reduction of the lower valve in young Anomia glabra is ev- 

 idently correlated with its conditions of environment. The byssal and retractor muscles 

 are functionally active in drawing down the upper valve, and when that valve is closed 

 the lower valve is completely protected. The reduction of the true adductor muscle as 

 described and the thinness of the right valve are doubtless correlated; for, on account of 

 its delicacy, the strong action of a normal-sized adductor would certainly break the valve. 

 At this early period the i-iglit valve ti-uly seems like a disap])earing organ. In later 

 gi-owth the right valve increases in thickness aud fi-equently extends beyond the object 

 of support, but it does not acquire the thickness and solidity of the left valve. 



I have stated that the right valve in young Anomia glabra is wholly com^^osed of 

 prismatic cellular tissue. This structure predominates throughout life in this valve, and 

 in the adult the i-ight valve entirely consists of prismatic cellular tissue with the excep- 

 tion of a thin layer of whiter porcellaneous tissue which surrounds the byssal foiTtUien 

 and point of insertion of the adductor muscle. The left valve, on the othei' hand, does 



