OSTREA. 385 



the corresponding part of the other valve is nearly flat and smooth (Plate LVIII, 

 figs. 3 — 5). In the larger specimens with a relatively small attached surface the 

 left valve (fig. 188) is more convex than the right, the latter becoming in some 

 cases nearly flat or even concave. The free part of the shell develops folds. 

 When the attached surface is relatively small the greater part of both valves 

 possesses strong radial folds which interlock at the margin of the valves. The 

 folds have more or less rounded summits, sometimes with small pointed projections, 

 which occasionally develop into spines. The interspaces are usually broader than 

 the folds. Well-marked growth-lines run parallel to the folded margin of the 

 valves. 



When only the margin of the shell is free no folds are developed, but the edge 

 of the shell becomes toothed (Plate LVIT, fig. 12) ; when a narrow strip is free 

 numerous small folds appear on it (Plate LV1I, fig. 13; Plate LVIII, figs. 1 — 

 4), forming a corrugated border to the smooth attached part of the shell. 

 Occasionally, even when the greater part of the shell is free the folds are indistinct 

 (figs. 183 — 185, 191). The direction of growth of the umbo, area, and ligament- 

 pit vary according to the position of the attached surface ; they may be in the 

 plane of the valves, or curve inwards or outwards, sometimes becoming nearly 

 perpendicular to the plane of the valves of the adult shell. The ligament-pit, area 

 and umbones may be at right angles to the length of the shell, or may curve 

 obliquely backwards or occasionally forwards. In some cases the area and 

 ligament-pit are higher than long, in others longer than high. 



The inner margin of the valves near the umbo often shows crenulations or 

 minute irregular ridges. The form of the adductor impression varies with that of 

 the shell ; it is vertically elongated when the shell is high, and more rounded when 

 the shell is longer. 



Affinities. — A number of the different forms of this shell have been described 

 as distinct species, since many writers have failed to recognise that the mode of 

 growth of the shell is determined mainly by the character and position of the 

 attached surface. The small sickle-shaped form (Plate LVI, fig. 18), with only a 

 small surface of attachment and the two valves similar, was named 0. pusilla by 

 Nilsson (1827), l 0. alseformis by Woodward (1833, pi. vi, fig. 1), and 0. Goldfussi 

 by Holzapfel (1889). Small forms which were attached for a longer period to a 

 cylindrical object, directed more or less nearly perpendicularly to the hinge (Plate 

 LVII, figs. 8—11, Plate LVIII, figs. 1, 2) and with the margin of the valve folded 

 and sometimes extended postero-ventrally were named 0. alseformis by Woodward 

 (1833, pi. vi, fig. 2), 0. Bruvni by Miiller (1851), 2 0. Peroni by Coquand (I860). 

 A form in which the shell was attached to a cylindrical object throughout except 



1 Hennig (1897), pi. i, figs. 14, 24. 



2 See also Hennig (1897), pi. i, figs. 7, 11, 12. 



