Prof. Huxley on Cephalaspis and Pteraspis. 319 



January C, 1858. — Major-General Portlock, LL.D., President, 

 in the Cliair. 



The following communications were read : — 



1. "On Cephalaspis and Pteraspis." By Prof. Huxley, F.R.S., 

 F.G.S. 



Of the four species originally included by Prof. Agassiz in the genus 

 Cephalaspis, two, C. Lloydii and C. Leioisii, are so different from the 

 others that the possibility of their proving genei-ically distinct is 

 hinted at in the ' llecherches sur les Poissoas Fossiles.' 



Subsequently M. Kner endeavoured to prove that these two species 

 are not fish-remains at all, but are the internal shells of a Cephalo- 

 pod, for which he proposed the generic name of Pteraspis. 



Rocmer has still more recently expressed the opinion that the 

 Pteraspides are Crustacea. These conflicting opinions clearly in- 

 dicate the necessity of revising and comparing anew the characters 

 of the different species of Cephalaspis and Pteraspis. And a still 

 greater interest is lent to the inquiry into the true nature of Pte- 

 raspis, from the fact that species of this genus are now known to 

 occur in undoubtedly Upper Silurian rocks. As the evidence stands 

 at present, they are, if fish, among the oldest (and nearly the very 

 oldest) representatives of their class. 



In undertaking this inquiry, the author of the present paper con- 

 sidered it desirable, in the first place, to determine with precision the 

 microscopical characters of the shield of Cephalaspis. This shield is 

 exceedingly thin, nowhere exceeding ^th of an inch in thickness on 

 the dorsal surface, and on the ventral suddenly thinning off a little 

 way from the margin into a mere membrane. 



The subjacent cranium appears to have been wholly composed of 

 cartilaginous or soft fibrous tissue ; for the " layer of fibrous bone," 

 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-brown colour in the immediate 

 neighbourhood of the animal substance ; the " fibres " of the supposed 

 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 lacunte, whose 

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

 successive layers, as in Mcgalichthys. Tlie lamella; and lacunae 

 disapjiear 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 ]Joints 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 i.s from tiie disposition of these vascular ramifications that the 

 appearance of distinct ossicles or scales, interlocking by sutures, 

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



