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SUPPLEMENT TO THE BRITISH 



Distinct, the shell doesnot adhere to marine objects by any portion of the substance of its 

 ventral valve (as is the case with Crania), but is attached by the means of a peduncle, 

 which passes through an oval foramen in the ventral valve, and the sculpture 

 observable on the surface of the dorsal valve is likewise reproduced on the ventral one. 

 In some species the lower valve is slightly conical, but in others is very thin, flat, and in 

 some instances gently concave, while the peduncle is so short, that the valve appears to 

 lie close to the object to which it is attached by the whole external surface of its lower 

 valve. Interiorly in recent species there appears to be very little or no flattened or raised 

 margin, but in several fossil forms, such as in Discina reflexa and D. Holdeni, a wide, 

 raised, oblique margin is seen, and the lower valve has quite the appearance, although 

 such may be more deceptive than real, of having adhered to shells and other objects by 

 the entire surface of its lower valve (see Sup., PI. X, figs. 6, 7, and Sup., PI. XI, 

 fig. 32 a). This appearance seems the more unaccountable from the fact that the oval 

 perforation for the passage of a peduncle is distinctly visible. In addition to the last 

 named peculiarities, we see on the inner raised surface of the valve regular radiating 

 lines, in addition to the concentric ones, which are in every probability vascular 

 markings. 



In all the Irish typical specimens of D. Holdeni that have come under my notice, the 

 shell was more or less crushed. Some examples were attached to a specimen of Astarte 

 deltilabrum from the Lias near Belfast, and I have several similar examples on an Ammonites 

 Valdani from the Middle Lias near Cheltenham. Mr. Beesley has found several specimens 

 attached to a valve of Dlagiostoma (Sup., PL XI, fig. 32), from the Lower Lias (Ammonites- 

 angulatus bed) of Harbury in Warwickshire (cutting of Great Western Railway). 



17. Discina Etheridgei, Dav. Sup., PI. X, figs 20, 20 a, b. 



Shell almost circular, or slightly longer than wide, conical, with vertex sub-central ; 

 length 12, width 11 lines. 



Obs. — Of this species the internal cast of the upper valve only has hitherto been 

 discovered, so that it is not possible to draw up a complete description of the shell. A 

 very fine example, stated to be from the Inferior Oolite of Nailsworth, in the Museum of 

 the School of Mines, shows the muscular and other impressions in an admirable state of 

 preservation. 



