398 EGBERT TRACY JACKSON ON THE 



shell becomes attached, the left valve, acting in !-ympathy with the right, become;- somewhat irregular iu growth ; Ijut is 

 less modifled thau the right by tlie new condition of cemented tisation. Life size, p. 324. Both specimens from the Eu- 

 ropean Tertiary, iu the Museum of Comparative Zoology. 



Fig. 5. Mulleria lobata, showing adult oyster-like form and at the tip of umbos a regular .\nodon-shaped shell preced- 

 iug the attached stage (after H. &. A. Adams), pp. 310, .■i24. 



Figs. 6-7. MvUeria lobata. Tips of the valves maguifipd to show the early stage, viewed from the ilorsal and left side 

 (after Broun), pp. 310, 324. 



Fig. S. Caprinula hoiss'ji, having a ch'ep, conical, rugose, attached valve and an arcuate, plain, free valve. European 

 Cretaceous, pp. 282, 319, 323. 



Fig. 9. Monopleura marcida, showing a highly uu)dirted attached vahe and a normal free valve. .Vmerican Cretaceous 

 (after AVhite (74), slightly modifled). pp. 319, 323. 



Fig. 10. Ca.prolina semistriata, .showiug a deep, rugose attached valve and shallow, plain, free valve. European Creta- 

 ceous. Compare with flg. 20, a young specimen of this species, pp. 319, 323. 



Figs. 11-12. Badiolites fleuriausa. having a highly modified, cornucopia-shaped, attached valve, fig. 12, and a less modi- 

 fied flatfish free valve, fig. 11. European Cretaceous, pp. 319, 324 (figs. 8 and 10-12, after d'Orbiguy). 



Fig. 13. Ostrea virginiana, section sho^ving the deep, concave, cemented valve, and flat free valve : l, lacuna" in the 

 sub-nacreous structure are filled with chalky lime ; ad, adductor muscle, which makes a purple scar at its area of insertion 

 in the valves. The black line indicates the jjositiou of the adductor at ditterent periods of growth, and is very noticeable 

 in the section, being marked by a continuous purple line. An instinctive section to show action of adductor muscle, liga- 

 ment, growth of shell and change of base of the muscle in successive growth. Buzzards Bay. Life .size, p. 320. 



Figs. 14-15. DavidsoneUa rugosa, sp. d'Orb., showing rugose, lower, attached valve (fig. 14) and a flat free valve (fig. 

 15), indicating that cementation in Brachiopods produces a similar result to tliat in Pelecypods. European Cretaceous 

 (after d'Orbiguy). p. 320. 



Fig. 16. Perna ephippium, adult, p|i. 327, 32!l, 383. 



Fig. 17. Perna isogomim, p. 327. 



Fig. 18. Perna vespertilio, p. 327. 



Fig. 19. Malleus alhus, adult, pp. 327, 383 (figs. 16-19 after Heeve, reduced). 



Fig. 20. Young Caprotina semistriata, from the Cretaceous. The lines of growtli are introduced according to judgment 

 to render the parts clear, as the specimen is too much eroded to show them. Compare ■with tlie adult, fig. 10, this plate. 

 Specimen No. 2902 in collections of the lluseum f)f Comparative Zoology. X 24 diam. pp. 319, 385. 



Fig. 20 drawn bv the author; all others drawn liy Mr. .1. II. Emert<m. 



PLATE XXVri. 



Fig. 1. Young Pej'jia ephippium, viewed from left side, shoiving,p, prodissoconch and succeeding dissoconch growth. 

 Compare with Pecteu, PI. xxviii, fig. 9 and Avicula, fig. 32, p. 330. The form of the shell is strikingly difl'erent from tlie 

 adult, PI. XXVI, fig. 16. Specimen from Museum of Comparative Zoology. X .")ii diam. (another view of this specimen, 

 see PI. xx-s-ni, fig. 11). pp. 328. 379, 383. 



Fig. 2. Young Perna ephippium. viewed from the right side, showing, p, prodissoconch and succeeding dissoconch 

 growth. The byssal notch originates at the close of the prodissoconch period in the initial stages of dissoconch growth. 

 The figure shows the margin of left valve extending beyond tlic right in the byssal area and several threads of the byssus. 

 Specimen from Boston Society of Natural History Collections. Cat. No. 2989. X 24 diam., p. 328. 



Fig. 3. Perna ephippium, same specimen and view as fig. 2, showing the prodissoconch. p, and the early stages of 

 growth of the right dissoconch valve. It is seen in figs. 2 and 3, that as in Pecten the left valve in early stages is larger 

 thau the right, and overhangs it at the umbos. X 87 diam., p. 328. 



Fig. 4. Y'oung Spondi/lus asperrimus, tip of right attached valve : p, prodissoconch : the succeeding early dissoconch 

 gi'owth is pecteniform with a deep byssal notch : as soon as attached the growth becomes irregular, the pecteniform shape 

 and byssal notch being eradicated. Compare with Hinuites. Pi. xxvii. fig. 3. Specimen in Boston Society of Natural His- 

 tory, Cat. No. 8494. X 50 diam., pp. 351, 390. 



Fig. 5. Young Plicatnln marginata, viewed from the left side, showing a prodissoconch. p. and succeeding dissoconch 

 growth. Specimen in Museum of Comparative Zoology. X 50 diam., pp. 325, 352, 392. 



Fig. 6. Y''oung Lima elliplica, showiug, p, prodissoconch, succeeded by the dissoconch growtli. Specimen iu Smith- 

 sonian Institution. From drawng made when at that Museum. Magnified, pp. 353, 388. 



Fig. 7. Oxytomainequivalvis, Sow.,umbonal region of right valve .showing p, prodissoconch, succeeded l)ya dissoconch 

 ■n'hich has a byssal notch in its initial stages of growth, and a triangular cartilage pit. Compare with Pecten. several figs., 

 PI. xxvni, and Avicula, fig. 33, p. 330. Specimen from the Middle Lias in the Museum of Comparative Zoology, Cat. No. 

 9222. X 50 diam., pp. 332, 382. 



Fig. 8. Chlamys islandica, hinge area of same specimen as PI. xxvm, fig. 14. viewed from tlie right side, showing, p, 

 prodissoconch, and the byssal notch originating iu the initial stages of dissoconch growth. Conijiare with Pecten irradians 

 PI. xxvm, figs. 10 and 13. The left valve is larger than tlie right. X 87 diam., p. 344. 



Fig. 9. Pecten irradians. Umbonal view from left side of a similar .specimen to that iu PI. xxvin, fig. 10, more enlarged, 

 showingp, prodissoconch, and the byssal notch originating at the close of the prodissoconch stage. X 87 diam., p. 342. 



Figs. 4 and 6 drawn bv the author: all others drawn by Mr. .T. H. Emerton. 



