Prof. Huxley on Ccphalaspis and Ttcraspis. 157 



which has been said to exist immediately beneath the shield, is in 

 reality nothing more than the matrix, which in these fossils, as in 

 others, is stained of a deep reddish-Ijrown colour in the immediate 

 neighbourhood of the animal substance ; the " fibres " of thesuj)poscd 

 bone are casts of the radiating semi-canals or grooves on the under 

 surface of the shield. 



The shield consists of three principal layers ; the outermost is 

 distinctly laminated, and contains numerous osseous lacunae, whose 

 long axes are disposed at a considerable angle to one another in the 

 successive layers, as in Megcdichthys. The lamella; and lacunte 

 disaj)pear in the middle and outer layers. The latter is arranged in 

 irregular tubercles, consisting of a substance very similar to the 

 " Kosmine " of Prof. Williamson. The inner openings of numerous 

 vascular canals are seen as points scattered over the inner surface of 

 the shield. These canals traverse the inner layer obliquely, and 

 then ramify in the middle layer in a very peculiar manner, described 

 at length in the paper. 



It is from the disposition of these vascular ramifications that the 

 appearance of distinct ossicles or scales, interloclsing by sutures, 

 which has been described, arises. The entire absence of any such 

 appearance of sutures on the inner surface of the shield is, indeed, 

 alone sufficient to prove that it is not composed of distinct scales. 



In the shield of Fteraspis three principal layers are similarly 

 discoverable : the inner is very distinctly laminated ; the outer, 

 almost wholly constituted by the characteristic " enamel-ridges," 

 consists of Kosmine. Vascular canals pass from the inner surface, 

 and ramify in the middle layer, terminating in ca^ca in the outer layer, 

 as in Ccphalaspis. 



But there are no osseous lacunae ; and the vascular canals com- 

 municate with large polygonal cells (which were either empty, or 

 more or less occujued by membranous substance in the recent state) 

 situated in the inner part of the middle layer. 



Specimens were exhibited in which these cellular cavities were 

 empty ; but ordinarily they are filled with the matrix, which then 

 assumes the form of polygonal prisms separated by the thin walls 

 of the cells. It is these prisms which have been mistaken for part 

 of the bony structure itself. 



On examining a thin section of one of M. Kner's specimens (for 

 which the author is indebted to the liberality of Sir P. Egcrton), the 

 structure, though much altered, showed sufficient similarity to that 

 of the specimens of C. Lloydii in the Museum of the Society to leave 

 no doubt as to the generic identity of the two. 



The microscopic examination of Pteruspis demonstrates its un- 

 questionably piscine nature ; and shows that, while in many respects 

 similar to Cephalaspis, the species included under Fteraspis are rightly 

 separated from the others. The leading distinctive characters of 

 the former are the absence of osseous lacuna?, — the cellular character 

 of the middle layer, — and the ridged and not tuberculated enamel. 



In conclusion, the author inquired into the evidence of the Ganoid 

 nature of the Cephalaspidce, and into the value of the relative and 



