396 ROBERT TRACY JACKSON ON THE 



Fig. IG. Young Ooniatites canaliculatus, var. gracilis: p, globular protoconch, succeeded by true conch.' pp. 291, 311. 



Fig. 17. Young Pteropod : p, ovoid protoconch (after Bronn). p. 291. 



Fig. 18. TfenJacM^Jtes ac«aru(s, Richter : p, protoconch (after Fischer), p. 291. 



Fig. 19. Developing Sycotopus canaliculatus, completed protoconch stage : h, heart ; /, foot ; », velum. Tlie shell 

 shows distinct lines of growth (after camera drawing by the author). X 24 diam., pp. 289, 291, 375. 



Fig. 20. Developing Sycotopus canaliculatus, a period much later than the close of the protoconch stage yet still re- 

 taining a velum, v, which is enormously developed, and has a ramifying muscular system, shown in the figure : /, foot; t, 

 tentacles; e, eyes and m, mouth. X 12 diam., p. 289. Figs. 19-20 from egg cases, Buzzards Bay, September, 1889 (after 

 camera drawing by the author). 



Fig. 21. Young CrepidiUa fornicata : p, spiraUy twisted protoconch, succeeded by the sub-conical nepionic growth. 

 Buzzards Bay. X 24 diam., pp. 292, 293. 



Fig. 22. Crepidula fornirata, a series of adult specimens growing on one another and with axes arranged in relation to 

 currents. An oyster is attached to the base of the colony. Buzzards Bay. Life size, p. 354. 



Fig. 23. Crepidula fornicata, much younger than fig. 2 1 : p, the protoconch, which is sharply defined, is succeeded by the 

 early nepionic growth. Nantucket, July, 1889. X 24 diam., pp. 292, 311. 



Fig. 24. Young Acmcea testudinalis var. alveus. Interesting as showing instead of a protoconch, a plug-like cicatrix, c, 

 formed where the protoconch dehisced. The cicatrix corresponds in form with the base of a typical protoconch. (Com- 

 pare with flg. 23). X 87 diam., pp. 292, 293. 



Figs. 7, 8, 17, 18, 23 and 24 drawn by the author; all others drawn by Mr. J. H. Emerton. 



PLATE XXIV. 



Fig. 1. Embryonic Ostrea virginiana, growing on a plate of glass from a drain-pipe trap. It is the completed prodis- 

 soconch stage and dimyarian : a. ad, anterior adductor muscle ; p. ad, posterior adductor ; g, gills, which consist of simple 

 filamentous processes (compare with fig. 6) ; v, velum ; pi, palps ; «, anus ; m, mantle border, which is broad and bears 

 no tentacles. The shell is wholly prodissoconch growth, no spat growth having taken place, and is cemented to the glass 

 by the ventral border of the left valve. The early straight hinge line similar to that of Ostrea edulis, flg. 25, p. 300, is 

 lost and superseded by a curved one. X 120 diam., pp. 282, 300, 308, 378. 



Fig. 2. Same specimen as flg. 1, viewed from the left side through the glass to which attached; lettering as in flg. 1. 



Fig. 3. Ostrea, the same specimen as flgs. 1 and 2, but three days older: pi, palps; ?■, rectum; g, gills, which at this 

 stage are composed of filaments connected at the tips by cross connecting bars ; m, mantle border, broad and not yet 

 possessing marginal tentacles ; p, the prodissoconch, which equals nearly half the entire height of the shell. Redrawn 

 from camera drawing by the author, p. 304. 



Fig. 4. Ostrea virginiana, young spat growing on glass: ad, adductor muscle; pi, palps. The body is permeated with 

 liver follicles; g, gills. Three gill lamella; are in view. A, A', B', for an understanding of which see fig. 8. In the early 

 growth of the right, outer gill i?', no con-esponding gill B exists on the leftside; st, stomach; int, intestine; o, anus; 

 h, heart; v, large blood vessel passing to mantle border; c.d, foramina through which afferent blood passes to heart; 

 m, mantle border fringed with tentacles; rm, radial muscles which retract the mantle; s, a sinuosity in the upper right 

 valve, exists at this poiut; the mantle is constantly retracted and all excurrent action takes place through this sinuosity 

 as indicated by arrows; p, prodissoconch. Redrawn from camera drawing by the author. X 12 diam., pp. 305, 307. 



Fig. 5. Gill plate from a young oyster of age of fig. 4, showing filaments joined by cross connecting bars. p. 305. 



Figs. 6-12. Diagrams of gills of developing Ostrea and adult Perna, Anomia and Pecten. The filaments in all are let- 

 tered the same so as to show the relation of the several diagrams : A, direct filament of inner gill of left side; a, reflected 

 border of same filament ; A', direct filament of inner gill of right side ; a', reflected border of same filament ; B, direct 

 filament of outer gill on left side; 6, reflected border of same filament; B' direct fllament of outer glU of right side; 6', 

 reflected border of same fllament. 



Fig. (!. Gills of young Ostrea virginiana, age of flgs. 1-2, this plate, pp. 301, 303. 



Fig. 7. Gills of Ostrea, later stage, age of fig. 3, this plate, p. 304. 



Fig. 8. Gills of Ostrea, still later stage, age of flg. 4, this plate, p. 305. 



Fig. 9. Gills of adult Ostrea, see PI. xxv, flg. 12, p. 30G. 



Fig. 10. Gills of adult Perna ephippium, see PI. xxv, fig. 11 pp. 304, 32G. 



Fig. 11. Gills of a.<\n\t Anomia glabra, \>. 358. 



Fig. 12. Gills of adult Pecten irradians, pp. 336, 341. 



Figs. 13-16. Ostrea edulis, early phylembryonic stage of shell growth; 13, viewed from the right side; 14, from the 

 ventral border; 15, from the left side; and 16, in end view. Compare with fig. 25, p. 300, an anatomical figure of about 

 the same age. The shell has a straight hinge line and only a slight indication of umbos. From the gill chamber of the 

 p.irent. X 120 diam., pp. 299, 312, 375. 



Fig. 17. Ostrea virginiana, completed prodissoconch stage, viewed from the upper right side, the specimen being al- 

 ready cemented to an object of support : r, right valve ; I, left valve. Compare with flg. 2G, p. 312. X 87 diam., pp. 312, 378. 



Fig. 18. A specimen slightly older than flg. 17, viewed from the anterior end: I, left valve; r, right valve; s, spat 

 growth, which has begun along the margin of the prodissoconch valves. X 87 diam., pp. 312, 315. 



Fig. 19. Young Ostrea virginiana. Tip of left valve from inside whorls of a dead Sycotopus, where it was ijreserved from 



'From Hyatt (27) after Saiulberger. 



