1857.] , huxley — cephalaspis and pteraspis. 267 



January 6, 1858. 



The Rev. Arthur W. Ingram, M.A., Hawington, and Timothy 

 Curley, Esq., Hereford, were elected Fellows. 



The following communications were read : — 



1. On Cephalaspis and Pteraspis. By Thomas H. Huxley, 

 F.R.S., F.G.S., Professor of Natural History, Government School 

 of Mines. 



[Plates XIV. XV.] 



The genus Cephalaspis (Agassiz) was originally established to in- 

 clude four species of Devonian fishes, — C. Lyellii, C. rostratus, 

 C. Lloydii, and C. Lewisii ; but the differences between the first and 

 the last of these species were so great, that the founder of the genus 

 himself suggested the probability of their future separation. 



The two groups of species are said by Prof. Agassiz to be con- 

 trasted not only by their forms, but also by their minute structure. 

 In regard to form, the cephalic disc of Cephalaspis Lyellii is stated 

 to possess an almost semicircular anterior outline, while its postero- 

 lateral angles are greatly prolonged backwards. The middle part of 

 the occipital region, Prof. Agassiz adds, is cut off almost square 

 (coupee presque carrement). As regards this last point, however, 

 my own observations are at variance with his description. 



Several specimens in the museum of this Society show that 

 the middle of the occipital margin is not truncated, but is greatly 

 produced backwards, the margins of the produced portion being 

 concave. The same peculiarity is clearly distinguishable in the 

 specimen of C. Lyellii now in the British Museum, and figured 

 by M. Agassiz, pi. 1. a. 2 : indeed the artist has faithfully depicted 

 the real contour of the occipital margin in the figure cited. The 

 well-known occipital spine is supported by this produced portion of 

 the disk. 



The discoid bodies, corresponding to all appearance with the 

 cephalic disc of C. Lyellii, upon which alone the species C. Lewisii 

 and Lloydii were established, differ widely from C. Lyellii, being 

 oval in contour and not prolonged into postero-lateral cornua. 



The structural differences observable in the disk of C. Lyellii 

 on the one hand, and of C. Lewisii and Lloydii on the other, are 

 thus stated by Prof. Agassiz : — 



" In C. Lyellii the head is covered with a pavement of polygonal 

 plates, altogether similar to that which covers the head of Ostracion. 

 Each plate is convex in the centre, and is marked by radiating grooves 

 ending at the margin in denticulations, by which the scales interlock. 

 These scales appear to be osseous and to have their external surface 

 enamelled. At the circumference of the disk they become con- 

 founded together, and the enamel presents wrinkles parallel to the 

 edge.' 5 Elsewhere these plates are said to be "true scales juxta- 

 posed." 



In the ' Recherches,' M. Agassiz describes " fibrous bones of the 



VOL. XIV.— PART I. T 



