398 ROBERT TRACY JACKSON ON THE 



shell becomes attached, the left valve, actiiijt in sympathy with the right, becomes somewhat irregular in growth; l)ut is 

 less raoclilied than tlie right by the new condition of cemented fixation. Life size, )). 3'Ji. Both specimens from the Eu- 

 ropean Tertiary, in the Museum of Compai-atire Zoology. 



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

 ing the attached stage (after H. &. A. Adams), pp. 310, 324. 



Figs. 0-7. Mulleria lobata. Tips of the valves magnified to show the early stage, viewed from the dorsal and left side 

 (after Bronn). pp. 310, 324. 



Fig. *<. Caprinnla hoissiji, liavius a deep, conical, rugose, attached valve and an an'uatc, plain, free valve. Euro))ean 

 Cretaceous, pp. 2,s2, 3i:i, 323. 



Fig. !•. Monopleura marcida, showiug a highly moditieil attached valve and a nurnuil frei' valve. .Vnierican Cretaceous 

 (after White (74), slightly modified), pp. 319, 323. 



Fig. 10. Caprolina semUlriata, showing a deep, rugose attaelied valve and shallow, plain, free \alve. Kuropean Creta- 

 ceous. Compare with fig. 20, a young specimen of this species, pp. 3 111, 323. 



Figs. 11-12. Radiolites fle^iriausa. having a highly modified, cornucopia-shaped, attached valve, tig. 12, and a less modi- 

 tied tlattish free valve, fig. 11. European Cretaceous, pp. 3iy, 324 (tigs. S and 10-12, after d'Orbigny). 



Fig. 13. Oslreo vir(/iniana, section showing the deep, concave, cemented valve, and flat free valve :/. lacunte in the 

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

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

 in the section, being marked by a contimious purple line. An instructive 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. Davidsonella rugosa, sp. d'Orb., showing rugose, lower, attached valve (flg. 14) and a flat free valve (fig. 

 15), indicating that cementation in Brachiopods produces a similar result to that in I'elecypods. European Cretaceous 

 (after d'Orbigny). p. 320. 



Fig 16. Perna ephippium, adult, pp. :')27, 3211, 3S3. 



Fig. 17. Perna isngonum, p. 327. 



Fig. 18. Perna vespertilio, p. 327. 



Fig. 10. Malleus albtis, adult, pp. 327, 383 (tigs. 16-10 after Heeve, reduced). 



Fig. 20. Young Caprotina semistriala, from the Cretaceous. The Hues of growth are introduced according to judgment 

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

 Specimen No. 2902 in collections of the Museum of Comparative Zoology. X 24 diam. pp. 310.385. 



Fig. 2(1 <lrawn liy the author; all others drawn I)y Mr. .T. II. Eniertou. 



PLATE XXVII. 



Fig. 1. Young Pec)i« ephippium, vieM'ed from left side, showing,;), prodissoconch and succeeding dissoconch growth. 

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

 adult, PI. XXVI, flg. 10. Specimen from Museum of Comparative Zoology. X 50 diam. (another view of this specimen, 

 see PI. xxviil, 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 gro'wtli. 

 The figure shows the margin of left valve extending beyond the 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 

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



Fig. 4. \o\\\\^ Spondylus asperrimus, X\[) ui right att.ached valve:;), prodissoconch; tlie succeeding early dissoconch 

 growth is pecteniform with a deep byssal notch; as soon as attached the growth becomes irregular, the pect(Miiforui shape 

 and byssal notch being eradicated. Compare with Hinnites, PI. xxvii, flg. 3. Specimen in Boston Society of Natural His- 

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



Fig. 5. Young Plicatula 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. Young Lima elliptica, showing, p, prodissoconch, succeeded by the dissoconch growth. Specimen in Smith- 

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



Fig. 7. Oxytoma inequivalvis, Sow., und^onal region of right valve showing j), prodissoconch, succeeded by a dissoconch 

 which has a b.vssal notch in its initial stages of growth, and a ti'iangular cartilage pit. (^onipare with Pecten, several figs., 

 PI. xx'STti, and Avicula, flg. 33, p. 330. Specimen from tlie Middle Lias in the Mustnim of Comparative Zoology, Cat. No. 

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



Fig. 8. Chlamys islandica, hinge area of same specimen as PI. xxvin, flg. 14, viewed from the right side, showing, p, 

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

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



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

 showing J), prodissoconch, and the Ijyssal notch originating at the close of the pi'odissoconch stage. X 87 diam. , p. 342. 



Figs. 4 and dniwii liy tlie author; all others drawn bv Mr. .T. H. Emerton. 



