PHYLOGENY OF THE PELECYPODA. 397 



all orudiiii; action : p, prodissocuiich ; s, spat UTOwtli; I, cartilasit; pit; /, tlaiii>e-like extension of margin of the shell ex- 

 teniling over the object of tixation, h, Ijorder of shell proper, exclusive of tlange. X 87 diani., pp. 315, 32i). 



Fig. 20. Yonng Oslrea vinjiniana, growing on glass, viewed from right side, showing p, left and riglit valves of 

 prodissoconch, and s, the first stages of spat growth. X 24 diani., p. 315. 



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

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



Fig. 22. Oryphma arcuala, umbos of shell showing flat area of early flxation in left valve and corresi)onding convex 

 area of right. Sp^cimsu from Lower Lias; in Mnsenm of Compar.itive Zoology. X 11 diam., p. 317. 



Fig. 2.1. Gryplicea cakeoln, var. nelirascensis, showing a flat area of early attachment in this highly arcuate species and 

 a prodissoconch, p. Jurassic of Wyoming, specimen Xo. 1881. U. S. National Museum. X 1 1 diam., ]i]). 313. 317. 



Fig. 24. Same specimen as tig. 23. viewed from in front. 



Figs. 3-4 and 22-24 drawn liy Jli'. .1. 11. Enirrton; flgs. 17-18 drawn by Mr. F. W. Cobb; all other Hgures drawn by 

 the author. 



PLATE XXV. 



Fig. 1. Young Ostrea cj'rj/iuiaiia, growing on glass, viewed from the riglit side : p, left and rii;ht valves of prodisso- 

 conch; Is. 2s. limits of rtrstand second spat or nepionic stages. 



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

 soconch; Is, 2s, limits of first and second spat or nepionic stages; I. margin of left valve, beyond which the right 

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



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

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



Fig. 4. Young Ostrea virgiiiiana viewed from right side, the specimen being attached to a shell : j>, jn-odissoconch, 

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



Fig. 5. Young Osln-a virgiiiiana separated from a glass slide on which it had grown and viewed fi'om right upper side : 

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

 the right valve where excurrent action takes jilace. Compare with left view of same specimen, fig. (>. X 8 diam., pp. 

 307, 31 li. 



Fig. (!. Same specimen as flg. 5, Ijut viewed from the left side, by which 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 aj-e marked in this valve as in the right valve, p. 316. Figs. 1-0. inclu- 

 sive, are of specimens from Buzzards Bay. 



Fig. 7. Young Oslrea virginiaua from Provincetown, Massachusetts, showing nepionic or spat stages of growth of 

 the right valve (Is, 2s, 3s, 4s, os). The first two stages were drawn from a younger specimen, as they were worn oft" 

 in the specimen at hand. X j^ diam. p. 3IG. 



Fig. 8. Ostrea viryiiiiana. Two adult individuals fixed to the same object of attachment. The left hand specimen is 

 growing in the normal position with the right valve uppermost, and the left valve lowermost. The right hand specimen 

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

 tive form of the valves is maintained. Buzzards Bay. X j diam., pp. 31G, 321. 



Figs. 9-12. Revolution of the axes, showing the relations of the axes of the body to the shell in typical Pelecj'pods, 

 and the serial mechanical relation of the same. Lettering in the figures : ap. ax, antero-posterioraxis, drawn through the 

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

 adductor nmscle;p. ad, posterior adductor; in, mouth; pZ, palps; a, anus; g, gills; pd, pedal muscles;/, foot; b, byssus ; 

 h, heart. 



Fig. 9. M;ia arenaria, p. 309. 



Fig- 10. Mudiola pUcatula, ]). &if.). 



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



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



Fig. f<, drawn by the kindness of Mrs. .1. W. Elliot ; all other figures drawn by the author. 



PLATE XXVI. 



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

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



Fig. 2. Cftrt»ios«)'ertn;6iVZa, showing an ostreaform, cemented valve and flatter free valve (after H. & A.Adams), p. 323. 



Fig. 3. Hinnites cortesi, right valve, showing an early nealogic, XTgular pecteuiform stage, possessing a byssal sinus, 

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

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



Fig. 4. Hinnites oortesi, left valve, showing an early regular pecteuiform shell, as does the right valve, flg. 3. When the 



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



