PHYLOGENY OF THP: PPXECYPODA. 399 



PLATE XXVIII. 



Fig. 1. Youuj; Peclen irracUaiis, viewed from tlie left side: (ul. adductor muscle; g, gills; i, intestine; a. amis; h, 

 heart; /, foot; p r, posterior retractor of the foot; Hi, mantle border bearing eyes, e; and alternating tentacles, t. The 

 shell bears a prodissoeonch. p. and is of about the age of tig. 12. this plate. Redrawn from camera drawini;- by the aiitlior. 

 X 40 diam., p. 333. 



Fig. 2. Pecten irradians, older than any other young Pecten ligured. Shows an active extensible foot; the mantle bor- 

 der bears a single eye in each plication of the shell and alternating witli each eye two tentacles (tentacles are omitted in the 

 right mantle lobe for the sake of clearness). Under the dorsal ear a special (irotrnsiou of the mantle border (pseudosi- 

 phon) exists, and at this point all excurrent flow takes place as indicated Ijytlie arrow. The shell shows no prodissoeonch, 

 but the smooth early nepionic stage is indicated. The plications ai'e as in the adult, flg. 37, p. 333, in number and form. 

 The hinge line is I'clatively long, and Init .slight ears are developed. X 16 diam., pp. 335, 345, 387. 



Fig. 3. Pecten irradians, diagram of a gill fllaraent of adult: a, direct border; a', reflected border; s, septum connect- 

 ing direct and reflected border of about every fifteenth filament : hj. blade cimimencing when se])tuni diminishes. Compare 

 -with PI. XXIV, tig. 12. p. 341. 



Fig. 4. Pecten, di;igramniatic section of adult, to show relations of mantle wall and tentacles : s, sliell ; llr/, liiranieut; 

 arf, adductor: m, mantle; mio, mantle wall; e, eye; t, marginal tentacles ; (j l, guard tentiicles. p. 338. 



Fig. 5. Fa'ces of Pecteu, seen in section and longitudinally, p. 340. 



Fig. 6. Pecten, mantle border of fig. 1 enlarged, showing cup-shaped eyes idternating with single i)apiUose tentacles; 

 no mantle wall or guard tentacles are yet developed on the mantle border, pp. 335, 337. 



Fig. 7. Young Pecten, diagram of foot. In foi'ming the byssal attachment it may lay hold of a foreign (il)ject at its tip 

 and at the point of the byssal cleft, arching up between those points as in the figure, p. 337. 



Figs. 8, 8a. Pecten, diagram to illustrate the swimming. Arrows indicate the several currents of water ejected in swim- 

 ming as described in the text. jj. o'i'.K 



Fig. 0. Young Pecten irradians, viewed from left side: p, prodissoeonch, succeeded l)y the early nepiouie stage of the 

 dissoconch. The form of the shell is distinct and differs from all later stages. Compare with Perna. PI. xxx'ii, fig. 1 , and 

 Avicula, flg. 32, p. 330. X 50 diam , pp. 333. 342. 379, 387. 



Fig. 10. Pecten, .same specimen as fig. 0. but viewed from the right side. The liyssal notch originates with tlie initi;d 

 stages of dissoconch growth, it is deep, and lias a marginal tooth. The left valve o\-erliangs the right on its free liorder. 

 Compare with PI. xxvii. fig. 3, and PI. xxix, tig. ti. pp. 328, 342. 



Fig. 11. Perna ephippium, same specimen as PI. xxvii, flg. 1, viewed from umboual region: I v, left valve; which is 

 slightly larger than rv, the right valve. The umbos of the prodissoeonch, p, are directed posteriorly. X 50 diam., p. 328. 



Fig. 12. Pecten irradians, older than flgs. 0-10, viewed from the left side : p, prodissoeonch, succeeded by sraootli 

 early nepionic growth which gives place to a jilicated later growth. The hinge line is proportionately longer than in earlier 

 stages, figs. 0-10, and is also relatively longer than in the adult, fig. 37, p. 333. Very slight indications of ears exist at this 

 stage. X 40 diam., pp. 345. 387. 



Fig. 13. Pecten, same specimen iis fig. 12. viewed from the right side. Uescriptiou of shell its in fig. 12. A deep liys- 

 sal notch exists in this valve, originating at the close of the prodissoeonch period. Four teeth exist on the border of the 

 notch instead of one as in fig. 10. At this age the left valve has ceased to he produced beyond the l.iorder of the right on 

 on its free margins, as in fig. 10. The shell as viewed from lioth sides ditiVrs markedly from earlier and later stages in ffu-m. 

 p. 345. 



.VU file specimens of Pecten irradians figured were collected at Buzzards Bay, August. 188S. 



Fig. 14. Young Chlamys islandica, viewed from left side, showing, p, prodissoeonch, and a smooth early nepionic 

 growth, succeeded by a plicated later growth. The plications are like those of the adult and mark a stage by their intro- 

 duction. Tlie hinge line is longer relatively to the width of the shell at this stage than in tlie adult as noted in Pecten ir- 

 radians, and has slightly developed ears. Specimen No. 272!I4, Smithsonian Institution Catalogue. X 23 diam., pp. 349 and 

 388. An enlarged umboual view of this specimen seen from the right side, vide PI. xxvii. fig. 8. 



Figs. 3-8 drawn by the author; all others drawn by Mr. .1. H. Emerton. 



PLATE XXIX. 



Fig. 1. Young An(jmia ijlahra. growing on glass, viewed from tlie leftside: ad, adductor mnscle; p r, posterior re- 

 tractor and a r, anterior retractor muscles of the foot; /, foot; pi, palps; a, anus; h, body permeated with liver fol- 

 licles; g, gills. Two gill lamelUu are seen from this side, A and b (compare with diagram. PI. xxiv, fig. 11) ; s, sheath 

 supporting gills; h, heart; v, v, lilood vessels; to, mantle border; p. prodissoc<nicli. p. 355. 



Fig. 2. Anomia, same specimen as fig. 1, viewed from the right side through the glass to which attached: ad. adduc- 

 tor muscle ; a r, anterior retractor muscle of the foot, attached to the left upper vahx, but seen through the glass ; hs, 

 calcifying byssal tip composed of separate discs of lime which unite to form parallel lines, as shown in the figure; /, foot; 

 pi, palps, showing slightly in this view; </, gills of which three lamelL-e, a, A', 6', are visible from this side (compare with 

 PI. XXIV, fig. 11) ; s, sheath which supports the gills; h, heart, a ventricle and auricle are visible from this side; a, anus; 

 c, ciEcum, highly produced ; m, mantle border ; rv. border of rig'ht valve which by a sinuosity encloses the byssus ; Iv, 

 border of left valve. Figs. 1 and 2 magnified about 20 diam. Redrawn from camera drawings by the author, p. 355. 



Fig. 3. Diagrammatic cross-section of flg. 1, to show action of the muscles; lettering as in fig. 1. p. 355. 



