Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/497

 viz. the Oxford clay, Cornbrash, &c. This is just the period in which the genus Gyrodus seems to have attained its maximum development. No species has yet been discovered in the Trias or Lias. A few occur in the Lower Oolites, from which period the genus expands enormously through the remainder of the Oolitic series, decreasing again in the Wealden, and finally disappearing in the Cretacean era. I have named this species Gyrodus Goweri in recognition of the great interest the Duke of Sutherland takes in all questions affecting the county of Sutherland, which has been remarkably shown by his establishing a museum at Dunrobin Castle for the reception of specimens of science and antiquity connected with the county, for the amusement and instruction of those who visit the charming scenery of the neighbourhood of Golspie.

2. Gyrodus coccoderma, Egerton.

The Museum of Practical Geology possesses a portion of a large Picnodont fish referable to the genus Gyrodus, from the Kimmeridge Clay of Kimmeridge. The specimen is a mere fragment, containing only a group of scales from the nuchal and dorsal region, above the lateral line, and before the dorsal fin. It measures 1 foot in length by 7 inches in depth. Judging from the proportions of other species of the genus Gyrodus, this fish, when perfect, must have been at least 30 inches in length, from the snout to the commencement of the tail, by 20 inches in depth, rivalling in size the great Gyrodus rhomhoidalis of the Munich Museum.

It contains twelve dorso -lateral series of scales, with eight scales in each row, measuring each 1 inch in antero-posterior diameter. They vary in depth and form according to the position they occupy in the series. Those nearer the top of the back are the shortest, and are lozenge-shaped ; the succeeding scales in each row become more elongated in descending order, so far as they are preserved. The scales of the flanks are unfortunately not present ; they would probably have been from 2-1/2 to 3 inches in length. The surface- ornament of the scales (fig. 3) is well seen, and is very peculiar. It consists of a multitude of symmetrical granules of hard and lustrous ganoine, like grains of millet-seed, irregularly scattered over the surface. On the nape and back they are thickly grouped, but more sparsely sprinkled over the other parts. Each granule maintains its form, even where they are most crowded, and has no tendency to coalesce with its neighbours to constitute the meandrine pattern so frequently found on scales with raised patterns. It differs in this respect from Gyrodus Goweri and Gyrodus rhomhoidalis, but somewhat resembles a little-known species from the Oolite of Stonesfield, named by Agassiz Gyrodus perlatus — the articulating processes of the scales forming the dermal ribs, and being very strong, and coarsely striated longitudinally. They measure half an inch in length. I have given the specific title coccoderma to this fish, from the characteristic features of the scales.