LAMELLIBRANCHIATA. 



surface is marked with concentrically-disposed foiiaceous ridges or 

 with roughly radial ribs. The umbo is not incurved, and below it 

 is a transversely striated ligamental pit. The Edible Oyster (O. 

 edulis) is a familiar example of this section of the genus, and essen- 

 tially similar forms are known to have existed in the Jurassic rocks. 

 In the sub-genus Alectryonia (fig. 558) both the valves are strongly 

 ribbed or folded, the edges of the valves being thus indented in a 



Fig. 560. — Exogyra aqicila. Lower Greensand. 



zigzag manner ; while the adductor impression is placed far back. 

 Forms of this type began to exist as early as the Trias, and the 

 recent O. crista-galli is a familiar example of a now living form of 

 the group. 



In the sub-genus Gryphcea (fig. 559), the two valves are of un- 

 equal length, the left valve fixed in early life, but often becoming 

 free in the adult condition, while its beak is prolonged and incurved, 

 being bent either forwards or backwards. The right valve, on the 

 other hand, is small in size, and is flat or concave in shape, looking 

 like a kind of operculum to the left valve. The species of Gryphcea 



Fig. 561. — Exogyra virgnla. Upper Oolites. 



are pre-eminently Jurassic and Cretaceous, but a few Tertiary and 

 Recent forms of the group are known. 



In the sub-genus Exogyra, again, the shell is very inequivalve 

 (figs. 557, 560, and 561), and is fixed by the substance of the thick 

 and concave left valve, the right valve being flat and resembling an 

 " operculum " in form. The beaks of both valves are rolled up, 

 being " reversed " — that is to say, turned towards the posterior side 



