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Show 1887.] CANAL-SYSTEM OF PTERASPIDIAN ITSHKS. 479 shield of Holaspis having been presented by its discoverer Dr. D. M . MacCullough, and the originals of Lankester's pl. i. fig. 8, pl. vi. fig. 6, having been acquired by purchase and bequest; and there are several other important specimens, likewise displaying in a greater or less degree the same peculiar superficial marks. With one exception, however, they afford no more precise information as to the character of the sensory lines thus indicated ; and the extreme rarity of the combination of circumstances by which a single example is made to throw further light upon the subject renders this fossil of unusual interest and value. I have lately met with it among a number of more or less broken shields obtained from the collection of the late Mr. E. Baugh, and the biological significance of the features it presents seems to render it worthy of some brief notice. The specimen in question is a fragmentary median plate, referable Fragmentary Median Plate of Shield of Pteraspis crouchii, Lower Old Red Sandstone, Herefordshire. [Brit. Mus. no. 42163 a.] to the cephalic buckler of Pteraspis crouchii, and is in the ordinary mineral condition of the Pteraspidian fossils from the Lower Old Red Sandstone of Herefordshire, whence it was derived. The striated outer layer is mostly removed, only occurring in small isolated patches, and the median " cancellated" layer1 is thus very completely exposed to view. But, unlike all other similarly abraded examples in the collection, this fossil shows not merely the innumerable small polygonal cavities, with their partitions, constituting the middle portion of the shield, but also a branching system of wide canals, which have no connection with these chambers, though distinctly ramifying through them. The latter have been most beautifully rendered evident by a dark infiltration of the oxides of iron and manganese (a kind of natural "injection"), and they are seen to have opened upon the external surface in a double series of orifices of considerable size. The "pits" or "depressions" described by Lankester, in fact, are proved to be really the openings 1 T H. Huxley, " On Ccphcdaspis and Pteraspis," Quart. Journ. Geol. Soc. vol. xiv. (1858) pp. 267-280. 32* |