On Cephalaspis and Pteraspis

The genus Cephalaspis (Agassiz) was originally established to include 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 contrasted 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 posterolateral angles are greatly prolonged backwards. The middle part of the occipital region. Prof. Agassiz adds, is cut off almost square (coupeé presque carrément). 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, pl. 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 being oval in contour and