118 MOLLUSC A FROM THE GREAT OOLITE. 



M. d'Orbigny regards this group as identical with Anatina. He believes that the 

 furrows upon the area are impressed by carinas, which were destined to support the 

 spoon-shaped processes of the hinge, and states that he has observed a chink or cleft at 

 the summit of the umbo, left by the spoon-shaped processes and by the internal osselet 

 of Anatina. 



M. Agassiz admits that these features would indicate an affinity with Anatina, but 

 directs attention to the elongated posterior side, to the cardinal area, and to the large 

 longitudinal ridges upon the sides of the valves. These characters, which are wanting 

 in Anatina, have induced him to retain his genus Cercomya. 



One character of the genus has not been alluded to by M. Agassiz. It possesses 

 an external semicorneous layer of test, which is furnished with radiating lines of tubercles, 

 as in Goniomya, Myacites, Gresslya, and in the recent Anatina. 



Anatina has not been found in the shelly beds of the Great Oolite. It occurs in 

 beds near to the base of the formation, in pale argillaceous buff-coloured limestones 

 and sandstone; it has also been found in the upper portion of the formation, associated 

 with Goniomya. 



M. Agassiz has sufficiently indicated the features which distinguish externally Cercomya 

 from Anatina. The interiors of the valves of the fossil species have not been seen, but 

 there is every reason to believe that they do not differ from Anatina. 



Anatina plicatella. Tab. XI, fig. C, 6 a. 



Testa transverse- elo/igatd, convexiusculd, latere postico elongato ; plicis concentricis 

 crcbris inconsjncuis, postice obsolctis. 



Shell transversely elongated, convex ; anterior side rather short, its upper border 

 sloping obliquely from the umbo ; posterior side more lengthened. Lateral longitudinal 

 plications closely arranged, distinct upon the anterior side of the shell, but disappearing as 

 they recede from it, so that the greater portion of the surface is nearly smooth. The very 

 delicate plications and general convexity of the valves are sufficient to distinguish it from 

 contemporaneous species. 



Height, 13 lines; length, 25 lines; diameter through both the valves, 9 lines. 



The figure nearly resembles that of C. aniica, Agassiz, tab. \\a, fig. 14, 15 ; but the 

 plications of that species are much larger and more continuous upon the sides. 



Locality. It occurs very rarely in Stonesfield Slate, on the south side of Minchin- 

 hampton Common. 



Anatina undulata, Sow. sp. Tab. XI, fig. 4. 



Sangtjinolaria undulata, Sow. Min. Con., t. 548, f. 1, 2. 

 — — Phil. Geol. York., 1, t. 5, f. 1. 



