BIVALVIA. 135 



obligingly presented to me by Professor E. Forbes, obtained by him in the iEgean 

 Sea, does not offer the slightest difference that could be considered as specific. 



The recent shell is nearly transparent in its young state, becoming a little 

 thickened when full grown, and then only the true form of the muscular impressions 

 can be observed, that upon the anterior side is somewhat elongated, and within the 

 mantle mark, but has not the band-like form of the true Lucina : the exterior is 

 smooth, with the exception of rather rough lines of growth, and in the centre of the 

 shell there is somewhat of a flattened space, which gives one side of a hexagon to 

 the ventral margin, there are two distinct depressions or sinuses on the posterior 

 side, and the dorsal margin of the shell is produced so as almost to cover over the 

 ligament, which might otherwise be called external, as it acts over a small fulcrum, 

 and opens the valve by its contraction ; there is one obtuse tooth in the right valve 

 at the anterior termination, or rather commencement of the ligament, with a 

 corresponding depression in the left valve, and the umbo curves a little towards the 

 anterior, over its large and deep lunule. 



Axinns angulatus of ' Min. Conch.' T. 315, is decidedly different, but the older 

 Tertiary shell "Lucina Goodkallii" from Hampstead, appears so strongly to resemble 

 our species, that I cannot consider their trifling differences to be more than the result 

 of locality, or of other conditions, and in examining many specimens of this shell 

 in the rich Cabinets of my friends, Messrs. Edwards and Wetherell, I could come 

 to no other conclusion, though all the specimens yet obtained have the two valves 

 so closely united, that their external characters alone are visible. The principal 

 difference appears to be in a rather more rounded outline to the older shell, which 

 has also less deeply produced folds or sinuses on the posterior side, but in the 

 examination of a specimen from Boom, in the Cabinet of Sir Charles Lyell, these 

 posterior sinuses were more strongly marked than in our Crag shell, with a rather 

 larger and deeper lunule, while the specimen itself exceeded in magnitude any of 

 my own, and judging from the figure and description of the Scandinavian shell Axinus 

 Sarsii, Loven, it does not appear to vary sufficiently to be considered specifically distinct. 



It is quoted by Nyst as a fossil from Bordeaux. 



2. Cryptodon ferruginosum, Forbes. Tab. XII, fig. 19, a, b. 



Ckyptodon rotundatum. S. Wood. Catalogue, 1840. 



Kellia eerruginosa. Forbes. jEgean Invert. Rep. Brit. Assoc, p. 192, 1843. 

 Artemis ? — Jeffreys. Aun. Nat. Hist., vol. xix, p. 313. 



Clausina — Id. vol. xx, p. 18. 



ABYSSICOLA. Id. • - - - p. 18. 



— Croulinensis. Id. - - - - p. 19. 



Lucina ferruginosa. Forb. and Hani. Hist. Brit. Moll., vol. ii, p. 60, pi. 34, fig. 1, 1849. 



Spec. Char. Testa minima, rotundato-ovatd, obliqud, subaquilaterali, tumidd, laevigata, 

 tenui, fragili ; latere postico obsolete unijrticato ; dente cardinali unico, obtuso. 



