BIVALVIA. 135 
obligingly presented to me by Professor E. Forbes, obtained by him in the Aigean 
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 Zwcima: 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. 
Awinus angulatus of ‘Min. Conch.’ T. 315, is decidedly different, but the older 
Tertiary shell “ Zucina Goodhallii” 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, Lovén, 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, 4. 
CryPpropon rotunpatuM. S. Wood. Catalogue, 1840. 
KELLIA FERRUGINOSA. Forbes. Aigean Invert. Rep. Brit. Assoc., p..192, 1843. 
ARTEMIS ? — Jeffreys. Aun. Nat. Hist., vol. xix, p. 313. 
CLAUSINA — Id. - a - vol. xx, p. 18. 
—  ABYSSICOLA. Id. - - - - p- 18. 
—  Crovuinensis. Id. - - - - pag: 
Luctna FERRUGINOSA. Forb. and Haal. Hist. Brit. Moll., vol. ii, p. 60, pl. 34, fig. 1, 1849. 
Spec. Char. Testé minimd, rotundato-ovatd, obliqud, subequilaterali, tumidd, laevigata, 
tenui, fragili ; latere postico obsolete uniplicato ; dente cardinali unico, obtuso. 
