1887.] CANAL-SYSTEM OF PTERASPIDIAN FISHKS. 479 



shield of Holaspis having been presented by its discoverer Dr. D. 

 M. MacCullough, and the originals of Lankester's pi. i. fig. 8, 

 pi. 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 hght 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 Pterasjns c9-otwhii, Lower Old 

 Eed 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" layer ^ 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, constitu- 

 ting 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 



^ T. H. Huxley, " On Cephalaspis and Pteraspis," Quart. Jouru. Geol Soc 

 vol. xiv. (1858) pp. 267-280. 



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