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furnished with a cleltoidal fissure, which is covered hy the cardinal process of 
the dorsal valve ; its length is about one-third that of the transverse diameter. 
The dorsal valve is a little more convex than the ventral valve, and is often 
slightly depressed towards the frontal margin. The external surface of both 
valves is covered with numerous fine radiating ribs, increasing in number by 
bifurcation or interposition. All these ribs may give rise to innumerable 
small spines, traces of which are most observable towards the margins of the 
shell. The internal structure of each valve is remarkable, and can be studied 
with ease on specimens from certain localities. On the interior of the ventral 
valve the dental plates extend for some distanee, and limit the muscular 
impressions; on a level with the deltoidal fissure, and between the dental 
plates, is a transversely striated impression, which has probably served as the 
attachment for the muscle of the peduncle. Immediately below this is a 
small median oval impression, on the sides of which are visible very much 
larger impressions composed of two distinct parts, the anterior of which pro- 
bably belongs to the cardinal muscle, and the other to the adductor muscle. 
Some ovarian and vascular impressions are often visible. The cardinal pro- 
cess is situated between two projecting teeth in the interior of the dorsal 
valve. The cardinal and peduncular impressions, reunited and ramifying, 
are divided in their median part by a small furrow which served as the 
attachment for the adductor. 
Dimensions. — The largest Australian specimen sent me is exactly 
twenty-five millimetres long, and about as broad ; its thickness is about six 
millimetres. 
Delations and Differences. — 0, Ilichelini resembles O. resupinata, 
Martin, and O. Keijserlingiana, de Koninck, in its external ornamentation, 
but it differs in its internal structure, and by the slight development of its 
hinge area ; it is also less susceptible to variations than O. resupinata, and 
never reaches the huge dimensions sometimes acquired by this last. In spite 
of all the care I have taken to compare Mr. O. A. Derby’s description of O. 
Denniana, and the figures he has drawn of it, with the numerous specimens 
of O. Miclielini, Leveille, at my disposal, taken from the Tournai Limestone, 
I can perceive no difference sufficient to prove that it is a distinct species. 
It appears to me to be merely a local variety. 
Horizon and Localities. — This species occupies a very widespread 
horizontal position, and is abundant in some localities. It is more common 
in the lower than in the upper beds of Carboniferous formations. It is of 
