154 
as 1847. The test being very thin, the shell could not possibly receive the 
strong muscular impressions which ornament the mould represented by the 
learned English Palaeontologist ; its muscular impressions are, on the con- 
trary, very superficial on all the moulds which I think are those of shells 
belonging to P. hrachytlicerus, .as it has been defined and described by 
Sowerby. Those of the adductor muscles are narrow, and consist of a few 
oblique ramifications ; the points of attachment for the cardinal muscles 
must have been very faintly marked, since they have left no trace at all. 
The rest of the internal surface shows some traces of elongated spiny 
tubercles which ornament the outer surface. The dorsal valve, of which I 
have examined two specimens, is nearly fiat in the visceral region, while its 
prolongation is recurved abruptly, and, fitting itself to the ventral valve, 
follows its trend up to the margins. Its external ornamentation consists of 
fine ribs like those of the ventral valve, with this difference, that, in place of 
some of the tubercles of the latter, there are punctures, elongated or rounded, 
according to their position ; and it is chiefly the marginal regions which 
bristle with spiniform, rather thick and long, tubes, irregularly inserted. The 
coneentric ridges are rather more apparent on the dorsal than on the ventral 
valve. The cardinal process is very small, and composed only of a trilobed 
tooth, not well marked. I could find no trace of a septum, or of muscular 
or vascular impressions, on the inner surface. 
Dimensions.— Tho, average length of the specimens sent me is tliirty- 
five millimetres ; the breadth, thirty-one millimetres ; and the thickness, 
twenty millimetres. 
Delations and Differences. — This species bears a great resemblance 
to P. scahriculns, Sowerby, and P. rngahis, Phillips. It differs from the 
former by being much more gibbous, and ])y the length of its prolonga- 
tions ; and from the latter by this last cliaracteristic, but especially by its 
longitudinal ribs, and its surface ornamentation of elongated tubercles. I 
am inclined to believe that Eig. 3, PI. XV of the Memoir by Mr. E. Etheridge 
on the fossils of Queensland,^ represents only the dorsal valve of a young 
individual of this species ; but I dare not affirm this without having seen 
the actual specimen figured l)y the learned Palaeontologist of the School of 
Mines, London. 
Quart. Journ. Geol. Soc., XXVIII, 1872. 
