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ribs. Some of these originate at the beak, and slightly thicken as they 
approach the margins, occasionally giving rise to short tubular spines ; 
between these ribs new ones arise by interposition. The test is perforated 
by innumerable tiny canals, the external orifices of which are perceptible as 
fine punctures covering the surface. In the interior of the ventral valve are 
some rather large dental plates, extending nearly parallel for some distance, 
bordering each side of the muscular impressions, which are separated by a 
rather thick median septum, thus giving rise to the formation of two elon- 
gated depressions, under which are seen traces of muscular impressions 
composed of several thin, scarcely perceptible, trunks. The dorsal valve 
possesses a tri-lol)ed cardinal process, placed between two very oblique well- 
developed brachial processes, beneath which is a rather broad median pro- 
tuberance, replacing the median septum, and serving to separate into two the 
four impressions of the adductor muscles. It is remarkable that among 
many specimens of this species I have not found a single one belonging to 
the variety which Professor Phillips named 0. connivens ; they all connect 
themselves with the typical variety first described by Martin. 
Dimensions. — The largest of the Australian specimens is four centi- 
metres long by five broad. 
Relations and Differences. — It would have been unnecessary to insist 
on the differences between this species and O. striatula, Schlotheim, if 
it had not been confounded with it by McCoy, who has also described and 
figured a young individual of it under the name O. australis. To confirm 
this statement, it will sufiice to compare the figure published by McCoy with 
those I have given. As to the difference between it and O. striatula, it con- 
sists not only in the internal structure, but still more in the general form of 
the two species, and in the nature of the striae or ribs with which their surface 
is ornamented. 
Horizon and Localities. — This species is very abundant in the various 
beds of the Carboniferous System, and seems to have existed throughout the 
whole Carboniferous Period. It is very common in Ireland, Scotland, and 
England, also at Vise and Tournay. In Australia it was found at Lewin’s 
Brook, Burragood, Colocolo, and Pallal, on the Gwydir. 
