198 MR. A. SMITH "WOODWARD ON [Mar. 1 7 



1. Note on some Dermal Plates of Homosteus from the Old 

 Red Sandstone of Caithness. By A. Smith Woodward, 

 F.Z.S.j of the British Museum (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. Scot- 

 land 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 know- 

 ledge of the subject is due chiefly to the researches of Hugh Miller^ 

 Asmuss^ Pander ^ and Traquair"* ; 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 millei-i 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. 



•0. H, Pander, ' Die Placodermen des deyonischen Systems' (1857), p.. 74, 

 pi. viii. figs. 2, 3, 6, 7. 



* E. H. Tra!quair, " Homosteus, Asmuss, compared with Coccosteus, Agassiz," 

 Geol. Mag. [3] vol. vi. (1889), p. 1, pi. i. 



