A flint cast of a Mytilus has been obtained from the gravel of Barnwell, near Cambridge, by Mr. Percevall, B.A., Trin. Hall, and entrusted to me for description.

Form elongated and narrow, attenuated anteriorly, with valves deep, and longitudinally striated. The anterior outline of the lips is straight, that of the posterior side a gentle curve, which is somewhat straightened towards the apex. The shell is about three times and a half as long as wide, and widest below the middle. The lateral outline of the valves is lanceolate, the greatest height being in the anterior third.

Australia offers both advantages and the reverse as a field wherein geology may be studied. Mesozoic strata being absent, or very sparingly developed, the variety of fossils is not extensive; but some of the phenomena observable are by no means devoid of interest.

The very limited portion of the Australian continent which I purpose describing is delineated upon sheet number 1 of the geological map lately issued by Mr. Selwyn,—a reduced sketch-copy of which is enclosed with the present communication (Pl. VI.).

By this map it will be seen that there are at least three classes of strata to be noticed—Silurian, Tertiary, and Volcanic. The site of the Victorian metropolis, together with the suburban townships, comprises the whole of these in various proportions. Melbourne proper, with the exception of its extreme western portion, which is situate on an older volcanic formation, is built almost wholly upon May Hill sandstones and soft laminated shales. The whole of this stratum has evidently been subjected to immense disturbance, it being a matter of no little difficulty to discover a patch many yards in extent within which the beds even approach to a horizontal position. The general inclination of the stratification more often verges on the perpendicular, whilst numerous contortions, folds, and doublings—theupper edges being cut away by denudation, at the same time that the lower are hidden far below the exposed portion—offer an interesting but withal most difficult problem to the geological student.