The Phyllolepis is a very striking genus of the same family, and has also been noticed at considerable length by the Swiss naturalist. The scales, or other plates, which covered the body of this fish are of enormous dimensions, being nearly half a foot in diameter, and rounded to an obtuse angle. What distinguishes them from all other scales, and particularly from those of the Holoptychius, with which they have certain external resemblances, is their extreme tenuity, consisting simply of a film of enamel spread over a thin osseous membrane, scarcely so thick as the blade of a knife, and varying from three to five inches in diameter. Their surface is smooth, or slightly marked with concentric wrinkles parallel to the edge of the scale. Two species of this genus have been found, one in the old red, and the other in the coal formation. In the Clashbennie sandstone only a few detached scales have been detected, but sufficiently well preserved to show the superposition, or imbrication, perhaps, in which they stood relatively to each other, the wrinkles serving as grooves by which their adhesion was more firmly effected. One decided characteristic of this organ in the _Phyllolepis concentricus_ is, that it is a little raised toward the middle, whence it again declines or sinks on all sides, after the manner of a roof. The sandstones flanking the hill of Kinnoul, and stretching along the left bank of the Tay, by Scone and Lethendy, appear to be a continuation of the Clashbennie beds, as also those occupying the ridges by Ruthven and Dupplin, where they assume much of the fissile character and micaceous aspect of the Carmylie flag-stone, but everywhere destitute of organic remains in the whole western district from Perth to Callander. The absence of fossils from particular beds has been accounted for in various ways. But even in the same series of rocks, and where there is no break in the continuity of the strata, it is a maxim of geology that the range of fossils is not always co-extensive with the mineral deposits. Then, as now, the explanation is, that the slightest physical changes affected the tastes and habits of the animal kingdom; the direction and strength of a current; the depth of water; the character and qualities of the sea-bottom; the force of tidal action; the season of the year, being, it is well known, singly sufficient to produce great differences as to the migrations and favorite haunts of almost every aquatic race. And hence it is laid down as a recognized principle in the science, that a particular bed of rock within certain limits is not to be excluded from its place in a system, and another substituted therein, by the mere presence or absence of a certain class of fossils. Individuals, too, will often outlive the family to which they belong, and be found in certain localities intermixed with the races of a higher group of rocks.—And these remarks are applicable to all the formations, less or more,