OSTREA. 383 



1002. OsTREA Mkkckyi, WoUemanu. Ibid., p. 51, pi. iii, fis^s. G, 7. 



— — SE5IIPLANA, J. p. J. Eavii. Mollusk. Danniark.s Kiidtafl. i, 



Liiiicllilir., p. llo, pi. iii, figs. 5, 

 7, 8. 

 1903. Alectryonia semiplana, E. Dacqur. Palaioutographica, vol. xxx, 2, p. 366, 



pi. xxxiv, fii^s. 1 — 4. 

 1905. OsTREA se:\iiplana, T. Wegner. Zeitscbi-. d. deutseh. geol. Gesellsch., vol. 



Ivii, p. 177, fig. 10. 



— — armata, Wegner. Hiid., p. 179, figs. 11-^13. 



? 1906. — — L. Kntmhedc. Palseontograpliica, vol. liii, p. 93. 



1908. — Peroni, p. a. Peron. Compte Reudii Assoc. Frauc. Avauc. Sci., 



vol. xxxvi (1907), p. 307. 



1909. — (Alectryonia) Peroni, M. Leriche. Anu. Soe. geol. Nord, vol. 



xxxviii, p. 67, pi. iii, figs. 

 20—27. 



— — semiplana, IF. J2o(/«/(ir. Bull. luternat. Acad. Sci. Cracdvie (1909), 



2, p. 690. 

 1911. — — Fritsch. Op. cit, Korycaner Schielit., p. 47, fig. 2U. 



1911. Alectryonia semiplana, K. Vogel v. Falcl-ensfeiii. Zeitselir. d. deutscli. 



geol. Gesellsch.. vol. Ixii, p. 559. 



prsrrijitio'i. — Form of sliell extremely variable, depending on the shape, size 

 and position of the surface to which the left valve is attached. In many specimens 

 tlie shell was fixed to a cylindrical ol)ject {rirli'miiitclla), hut in other cases to a 

 flat or .slightly curved surface. 



In the larger, fully grown specimens, in which the attached surface is relatively 

 small and mainly in front of the umbones, being either parallel or oblique to the 

 hinge, the shell is more or less triangular, or four-sided, or rounded, and its height 

 is then often greater than its length (Plate LVII, fig. 7; Text-figs. 183—192). 



When the shell is attached to a cylindrical body the length of which is at right 

 angles to the hinge-line, the height of the shell is, at any rate during the attached 

 stage, much greater than its length, and the front and back margins are more or 

 less nearly parallel (Plate LVII, figs. 8, 12), but subsequently the postero-ventral 

 [)art may grow in a posterior direction (Plate LVII, fig. 11). When the length 

 of the attached body is parallel or nearly parallel with the hinge-line, and posterior 

 to the umbo, the shell becomes much longer than high and extremely ineqnilatei-al 

 (Plate LVII, figs. 1 — 3). If the attached surface is directed obliquely backwards 

 from the umbo the shell becomes oblique and inequilateral. In small specimens 

 having a very small attached surface the shell becomes sickle-shaped and the two 

 valves similar in form (Plate LVI, fig. 18). 



During the period in which the left valve is attached to a cylindrical object the 

 corresponding i)art of the right valve takes a similarly convex foi-m and remains 

 without folds; when the left valve is attached to a flat or slightlv curved surface 



