354 



CRETACEOUS LAMELLIBRANCHIA. 



So that this type occurs at three horizons : Aptian, Cenomariian, and Senonian, 

 and its features are obviously due to the harge size and rounded form of the 

 attached surface; the shell was attached for such a long period that when it 

 eventually grew free it was unable to develop its usual elongate form, and its 

 ultimate outline was only a little different from that of its attached surface. 

 The coarseness of the ribs varies in the same way as in the elongate forms. In 

 some places, as, for instance, in the Cenomanian of Gamighiigel in Saxony, the 

 shell attains a great thickness, but the elongate forms likewise sometimes become 

 very thick. 



Figs. 135-138. — Ostrea dihti-iana, L135, zone of Molasier sulgloboswt, Cherry Hintou. Kijjht valve. 13G-13S, 

 Base of Chalk Marl, Haslingfield, Cambs. 136, 137, Right valves. 138, anterior view of 137. 

 Sedgwick Museum. All x ^. 



0. santonensis, d'Orbigny, and 0. carantonensix, d'Orbigny, are similar to 

 d'Orbigny's 0. diluviana, and are included by Geinitz in that species. 



Bemarls. — The shell varies greatly in its curvature, some examples being only 

 slightly bent, while others form almost a complete volution (fig. 123), and between 

 these extremes every gradation is found. The variation in the coarseness of the ribs 

 seems, in some cases at any rate, to be connected with the nature and depth of 

 the sea-floor on which the individuals lived ; for example, most of the specimens 

 found in the Chalk Marl have coarse ribs, but those found in the Cenomanian 

 sandstone of "Wilmino-ton in Devon have smaller and more numerous ribs. Occa- 

 sionally folds or ribs are developed on the posterior ear (fig. 135). The long 

 regular outgrowths from the margin of the valves (fiig. 122) occur in specimens 



