Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/671

Rh duction of the covered areas is proportionally longer and more acute. The scales of the other coherent patch are smaller in size, and more obliquely rhomboidal in form, as regards the exposed surface ; they are further distinguished by the absence of the articular peg of the upper margin, and by the much greater narrowness of the covered areas, which are not specially produced upwards and forwards. Their thickness is also very considerable, being no less than $1⁄8$ inch in one of these scales entirely detached from the matrix, and measuring about $1⁄2$ inch in breadth. A difference is also observable in the sculpture of these posterior scales, viz. a tendency of the ridges to coalesce in a reticulating manner towards the posterior, superior, and interior-inferior obtuse angles of the scale, so as to interrupt the intervening furrows in these two regions, converting them more or less into pits.

The specimen also exhibits, as already mentioned, many detached and broken-up, transversely jointed fin-rays, some of which attain a breadth of $1⁄8$ inch; the length of their joints is somewhat less, but varies in different rays. These rays are all seen only from their internal non-ganoid surfaces.

As already stated, it is quite evident that the fish to which these remains belonged is a member of the family Palæoniscidæ; and the form and thickness of the scales, with their very large anterior covered area, and the nature of their sculpture, along with the peculiar tubercular ornamentation of the cephalic and shoulder-bones, point out, as it seems to me, that Acrolepis is the genus to which it should be referred. The scales of the Permian A. Sedgwickii are quite similar in shape, though proportionally smaller and with fewer ridges. The tendency to reticulation of the ridges on certain parts of the posterior scales of the Lanarkshire fish reminds us also of the peculiar sculpture which is characteristic of the entire scale and over the whole body of A. exsculptus. The narrow ventral scales are, indeed, undistinguishable from the one from the Carboniferous Limestone of Derbyshire contained in the Cambridge Museum, and figured by Mc Coy as Acrolepis Hopkinsii ; but if that be the same, as I believe it to be, with a fish from the Millstone Grit of Hebden Bridge, of which several beautiful fragments are in the collection of Mr. John Aitken of Bacup, it is a distinct species, and differs from A. Rankinei in other respects.

In conclusion, if all definition of Gyrolepis as a genus is at pre- sent impossible, if the diagonal ridged scale-ornament, supposed to be characteristic of it, is also characteristic of the scales of many species belonging to various other genera, such as Acrolepis, Elonichthys, Rhabdolepis, Cosmoptychius, &c, and if the "Gyrolepis" Rankinei of Agassiz be referable to Acrolepis, then there is, as I have maintained above, no longer any justification for the retention of the name "Gyrolepis" in our lists of British Carboniferous fossils.