PHYLOGENY OF THE PELECYPODA. 399 



PLATE XXVIII. 



Fig. I. Young Pecteu irvadiaiis, viewed from tlie left side: nd, iidductor muscle; g, gills: t, intestiue; a, amis; ft, 

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

 shell bears a prodissoconeh.p, and is of aliont tlii'age of tig. 12. this plate. Kedrawn from camera drawing by the author. 

 X 40 diam. , p. 333. 



Fig. 2. Pecten inxidians, older than any other young Pecten figured. Shows an !ictive extensible foot : tlie mantle lior- 

 der bears a single e.ye in each plication of tlie shell and alternating with each eye two tentacles (tentacles are omitted in the 

 right mantle lobe for the sake of clearness). Under the dorsal ear a special protrusion of the mantle l)order (pseudosl- 

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

 but the smooth early nepionic stage is indicated. The plications are as in the adult, fig. 37, p. 333, in number and form. 

 The hinge line is relatively long, and Init slight ears are developed. X 16 diam., pp. 33.5, 345, 387. 



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

 ing direct and reflected l)orderof about every fifteenth filament ; bt, blade commencing when septum diminishes. Compare 

 with PI. XXIV. fig. 12. p. 341. 



Fig. 4. Pecten, diagrammatic section of adult, to show relations of mantle wall and tentacles : s, shell; lig, ligament; 

 ad, adductor: m. mantle; mw, mantle wall ; e, eye; t, marginal tent;icles ; ij l, guard tentacles, p. 338. 



Fig. .">. Fajces of Pecten, seen in section and longitudinally, p. 340. 



Fig. ti. Pecten, mantle border of fig. 1 enlarged, showing cup-shaped eyes alternating witli single papillose tentacles; 

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



Fig. 7. Young Pecten. diagram of foot. In forming the byssal attachment it may lay hold of a foreign oljject 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. Pecteu, diagram to illustrate the swimminff. Arrows indicate the several currents of water ejected in swim- 

 ming as described in the test. p. 3311. 



Fig. !t. Young Pecten irradians, viewed from left si(h' : ;). prodissoconch. succeeded by the early nepionic stage of the 

 dissocouch. The form of the shell is distinct and ditl'ers from all later stages. C'oin|)arc with Periia. Pi. xxvii, flg. 1. and 

 Avicula, flg. 32, p. 330. X 5(1 diam , pp. 333. 342. 370, 387. 



Fig. 10. Pecten, same specimen as fig. it, but viewed from the right side. Tlie liyssal notch originates with the initial 

 stages of dlssoconch growth, it is deep, and has a marginal tooth. The left valve (nerhangs the right on its free border. 

 Compare with PI. xxvii, flg. 3, and PI. xxix, flg. (i. pp. 328, 342. 



Fig. 11. Perna ephippium, same specimen as PI. xxvu, fig. 1, viewed from umbonal region: I v, left valve; which is 

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



Fig. 12. Pecten irradians, older than figs. 11-10, viewed from tlie left side : p, prodissoconch, succeeded by smooth 

 early nepionic growth which gives place to a plicated 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 as fig. 12, viewed from the right side. Description of sliell ;is in tig. 12. A deep l)ys- 

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

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

 on its free margins, as in flg. 10. The shell as viewed from both sides differs markedly from earlier and later stages in form, 

 p. 345. 



All the specimens of Pecten irradians figured were collected at Buzzards B:iy. August, 1888. 



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

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

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

 radians, and has slightly developed ears. Specimen No. 27204, Smithsonian Institution Catalogue. X 23 diam., pp. 340 and 

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



Figs. 3-8 drawn by the author: all others drawn by Mr. .T. H. Emertou. 



PLATE XXIX. 



Fig. 1. Young Anomia ylabra. growing on glass, viewed from the left side: ad, adductor muscle; p r, posterior re- 

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

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

 supporting gills; h, heart; v, v, blood vessels; in. mantle border; p, prodissoconch. 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 valve, but seen through the glass ; bs, 

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

 pi, palps, showing slightly in this view; y, gills of which three lamella', a, A', b', are visible from this side (compare with 

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

 c, ca;cum, highly produced; in. mantle border; rv. border of right 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. 3,55. 



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



