84 



Ostrea arcuata, Deshayes, 1849, Traite Elementaire de Con- 



chyliologie. PI. 56, f. 8, 9. 

 Ostrea arcuata, D'Orbigny, 1850, Prodrome de Pal^ontologie Strati- 



grapWque. Vol. I, p. 220. 

 It is, in fact, the shell invariably figured as the best type of the 

 subgenus to which it belongs, and cannot fail to be recognised from 

 the rudest figure, or from the following description, which is here 

 somewhat amplified from Sowerby. 



Specific Character. 



" Elongated, very involuted, right side" presenting a more or less 

 " strongly marked, or an obscure lobe," (when viewed with the smaller 

 valve placed downwards, and the umbonal portion turned away from 

 and at right angles to the front of the observer ;) " lesser valve 

 oblong," *' externally concave." 



This description applies only to the ordinary adult form, than which 

 none would appear at first sight to be more easily determinable, but 

 the following list of what are considered by some authors of repute to 

 be distinct species, and merely synonymes of one, by others, will at 

 once give an idea of the notable modifications and changes of form, of 

 which this species is susceptible, the shells named in it representing 

 every imaginable gradation between the outlines of the Common 

 Oyster, and those of the most perfectly developed G. arcuata. 



G. MaccuUochii of Zieten Table 



G. laeviuscula ,, 



G. ovalis ,, 



G. MaccuUochii of Sowerby. ... 



G. gigantea „ 



G. obliquata, Goldfuss. 



G. obliquata of Sowerby. 



O. irregularis, Goldfuss. 



O. laeviuscula, ,, ... ... „ » m i, 



O. ungula, Miinster, Handbuch, jeune. „ 325. 



O. semicircularis, Roemer. 



O. irregularis, D'Orbigny, 1853. Prod. VI. p. 238. 



O. intermedia, Terquem. 



G. depressa, Phillips. 



G. lobata, Buvignier. 



Although Oysters have been found in much older formations, as 

 exemplified by the unique specimen of Ostrea nobilis, from the 

 carboniferous limestone of Belgium, which may be seen in the British 

 Museum, with others from the Triassic " Saliferian" of St. Cassian, 



