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Show 198 MR. A. SMITH WOODWARD ON [Mar. 1/ 1. Note on some Dermal Plates of Homosteus from the Old Red Sandstone of Caithness. By A. S M I T H W O O D W A R D , F.Z.S., of the British M u s e u m (Natural History). [Received March 2, 1891.] So much is now known concerning the dorsal shield of the large armoured fish Homosteus, from the Old Red Sandstone of N. Scotland and N.W. Russia, that little remains to be added beyond points of detail concerning the form of the various plates. The ventral shield, however, is still unrecognized, and much has yet to be determined in connection with the facial bones. Existing knowledge of the subject is due chiefly to the researches of Hugh Miller1, Asmuss2, Pander3, and Traquair4; and further advances can only be made by the discovery of additional specimens. For one such discovery, which makes known a few novel features of interest, the writer is indebted to Mr. Donald Calder, of Thurso, who has recently forwarded to him an associated group of five dermal plates of Homosteus milleri from the Thurso flagstones. The three occipital plates are isolated and beautifully exhibited from the visceral aspect; a smaller, bilaterally symmetrical plate, also exposed from the visceral face, seems to be the anterior median ventral element; and another plate, with an adjacent fragment, is most probably one of the anterior ventro-laterals. The median occipital is shown in the accompanying drawing, fig. 1, p. 199, the left lateral occipital in fig. 2, the anterior median ventral in fig. 3, and an impression of the supposed left anterior ventro-lateral in fig. 4 ; all the figures being of one quarter of the natural size. It has long been known that the median occipital plate in Homosteus overlaps the lateral occipitals to an enormous extent, but the precise limits of the great facette on each side have not been so clearly exhibited as in the new specimen (fig. 1,/). Except in the hinder two thirds of the anterior half, the overlapping surface is more extensive than the exposed visceral face ; and in front, where the bone is very robust, it exhibits a pair of broad facettes (/), distinct from the lateral pair and thus evidently overlapping the central plates immediately in advance. The exposed median portion exhibits a longitudinal ridge arising shortly in front of the occipital border and soon bifurcating into a symmetrical pair of ridges, which 1 H. Miller, ' Footprints of the Creator (1849), p. 70, figs. 24, 27-29, 36, 37, 39-41. 2 H. Asmuss, ' Das vollkommenste Hautskelet der bisher bekannten Thier-reihe' (Inaug. Dissert. Dorpat, 1856), pp. 8, 35. 'C. H . Pander, 'Die Placodermen des devonischen Systems' (1857), p. 74, pi. viii. figs. 2, 3, 6, 7. 4 R. H. Traquair, " Homosteus, Asmuss, compared with Coccosteus, Agassiz," Geol. Mag. [3] vol. vi. (1889), p. 1, pi. i. |