XLVII 



ON CEPHALASPIS AND PTERASPIS 



Geol. Soc. Quart. Joiirn., vol. xiv., 1858,//. 267-280 

 [Plates XXXI. and XXXII.] 



The genus Cephalaspis (Agassiz) was originally established to 

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

 Lloydii, and C. Leivisti; 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. i. «. 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 



