PHYLOGENY OF THE PELECYPODA. 397 



all eroding action : p. prodissocoiich ; s, spat growth ; I, cartilage pit ; /, tlange-likc extension of margin of the shell ex- 

 tending over the object of tixatiou; h, border of shell proper, exclusive of flange. X 87 diam., pp. 315, 329. 



Fig. 20. Yonng Ostrea virginiana, growing on glass, viewed from right side, showing p. left and right valves of 

 prodissoconch, and s, the tirst stages of spat growth. X 24 diam., p. 315. 



Fig. 21. Same specimen as fig. 20, viewed from the left side through the glass to whicli attached : p, prodissoconch; 

 s, spat growth, p. 315. Figures 17 to 21 are of specimens from Buzzai'ds Bay. 



Fig. 22. Gryphcea arcuata, umijos of .shell showing flat area of early fixation in left valve and corresijonding convex 

 area of right. Specimen from Lower Lias; in Museum of Comparative Zoology. X 11 diam., p. 317. 



Fig. 2.^. Gryphcen calceoln, var. nebrascensis, showing a flat area of early attachment in this highly arcuate sijecies and 

 a prodissoconch, p. .Turassic of Wyoming, specimen No. 1881, U. S. Xational Museum. X 11 diam., pp. 313. 317. 



p-fg. 24. Same specimen as fig. 23, view'ed from in front. 



Figs. 3-4 and 22-24 drawn by Jlr. .T. II. Emerton ; flgs. 17-lS drawn by Mr. F. \V. Cobb; all otlier figures drawn loy 

 the author. 



PLATK XXV. 



Fig. 1. Young Ostrea Oi(v/iUi(()i(«, growing on glass, viewed from the right side: ;>, left ami riglit valves of prmlisso- 

 c<nich ; Is, 2s, limits of Hrst and second spat or nepionic stages. 



Fig. 2. Same specimen as flg. 1, viewed from the left side through the glass to wliieh attaclied : p, left valve of prodis. 

 socouch; Is, 2s, limits of first and second spat or nepionic stages; I, margin of left valve, lieyond whicli the right 

 (upper) valve extends. Figs. 1 and 2, X 24 diam., p. 315. 



Fig. 3 Young Ostrea virginiana, viewed from the hinge area: p, prodissoconch; U. left valve; rv, right valve; s, early 

 spat growth. The left valve has not yet flattened to the object of attachment. X 120 diam., p. 315. 



Fig. 4. Y'oung Ostrea virginiana viewed from right side, the specitneu being attached to a shell: /), prodissoconcli, 

 succeeded by s, the early spat or nepionic growth. X 98 diam., pp. 311, 315. 



Fig. 5. Y''oung Ostrea virginiana separated f rofn a glass slide on which it had grown and view'ed from right u|iperside : 

 p, prodissoconch ; Is, 2s, 3s, 4s, four spat or nepionic stages of growth cleai-ly indicated in this specimen ; si, a. sinuosity in 

 the right valve where excurrent action takes place. Compare with left view of same specitneu. fig. (!. X 8 diatn. , pp. 

 307, 31(!. 



Fig. (i. Same specimen as tig. 5, liut viewed from the left side, by wliich it was attached to the glass before removal. 

 The left valve is seen to overhang the right on its free borders ; at si, the excurrent sinuosity of the right valve is very 

 noticeable in this figure, and four nepionic stages are marked in this valve as iti tlie right valve, p. 31(J. Figs. 1-0, iticlu- 

 sive, are of specimens from Buzzards Bay. 



Fig. 7. Young Ostrea virginiana from Provincetown, Massachusetts, showing nepionic or spat stages of growth of 

 the right valve (Is, 2s, 3s, 4s, .5s). The first two stages were drawn from a younger specifiien, as they were worn of!' 

 in the specimen at hand. X } diam. p. 316. 



Fig. 8. Ostrea virginiana. Two adult individuals fixed to the .sanfe object of attachment. The left hand specitneu is 

 growing itt the normal position with the right valve uppermost, and the left valve lowermost. The right hand specitneu 

 is growing in a reversed position with the right valve lowermost, and the left valve, uppermost; in both the satiie rela- 

 tive form of tlie valves is ntaintained. Buzzards Bay. X § diam., pp. 316, 321. 



Figs. 9-12. Revohitioti of the axes, showing the relations of the axes of the Ijody to the shell in typical Pelecypods, 

 and the serial mechanical relation of the saute. Lettering in the figures : ap. ax, antero-posterior axis, drawn througli the 

 mouth and middle of the posterior adductor tnuscle ; h. ax, hinge axis, the axis of motion of the valves ; a. ad, anterior 

 adductor muscle ; p. ad, posterior adductor ; m, month ; pi, palps ; a, attus ; g, gills ; pd, pedal muscles ; /, foot ; b, byssus ; 

 7i, heart. 



Fig. 9. il/j/a arenariit, p. 309. 



Fig. 10. Modiola plicatula, \). '^il'.K 



Fig. 11. Perna ephippium, pp. 309, 329. 



Fig. 12. Ostrea virginiana, pp. 305, 308, 327. 



Fig. 8, drawit by the kindness of Mrs. .T. W. Elliot; all other figures drawit by the author. 



PLATE XXVI. 



Fig. 1. Spondyhis hippuritarum , having a highly modified, deep, attached valve and less ttwxlitied free valve. European 

 Cretaceous (after d'Orliigtiy). pp. 319,323,351. 



Fig. 2. C/iaHJ0SJ?'enai6ifZ«, showitig an ostreafortn, cetneuted valve and flatter free valve (after H. & A. Adams), p. 323. 



Fig. 3. Semites cortesi, right valve, showing an early nealogic, regular pecteniform stage, possessing a byssal sinus, 

 which is succeeded, when the shell becomes attached to a foreign body, by an irregular ostreaform growth, possessitig no 

 byssal sinus, and losing almost entirely the features characteristic of early stages, pp. 282, 293, 294, 324, 350 and 390. 



Fig. 4. Hinnites cortesi, left valve, showing an early regitlar pectettifortn sliell, as does the right valve, fig. 3. When the 



MEMOIRS BOSTON SOC. NAT. HIST., VOL IV. 53 



