ROBERTS — DISTRIBUTIOX OF CEPHALASPIS AKD PTEEASPIS. 105 
among fish-families can be decided. Probably a kinship existed 
between the two chief forms of ichthyic test of Cephalaspis and 
Pteraspis, and it is most likely that our noble friend, the sturgeon 
{Acijjeiise)'), will have to own them of his family ; for, as Prof. Hux- 
ley has lately pointed out, they bear, in shape and arrangement of 
head-plates, a great afl&nity to the genus Spatidaria, a North 
American attache of our larger and caviare -giving fish. 
It may help the comprehension of those who are unfamiliar with 
the osseous head-shields of these old ganoid fishes, if I sketch the 
two forms whose acquaintance will be most easily made by exploring 
collectors, Cephalaspis and Pteraspis. Form of shell is a very 
deceptive guide both in fish and crustacean life ; indeed, if we made 
our affinities from this alone, one gi^eat genus would include many 
species of both orders, for the shape of Cephalaspidean bucklers is 
copied almost literally by several Crustacea. A new Harpes from the 
Silurian limestones of Oesel, figured by Eichwald, agrees not only 
in shape of head, but even a position of the eyes with Cephalaspis ; 
while it would be a matter of serious concern where to draw the line 
between the head-plates of Eurypteris and Cephalaspis. But it is 
from the closest and most minute examination that species and even 
families are determined among those which lived during the infancy 
of vertebiated life. 
Pteraspis Imdensis, the oldest fish with which we are at present 
acquainted, was found in the Lower Ludlow mudstones at Churchill 
Quarry, near Leintwardine, and has been well described and figured, 
side by side with its ally, Pteraspis truncatus, from the Upper Ludlow 
rocks, by Mr. Salter, m the "Annals of Nat. Hist." for July, 1859. 
VOL. IV. 0 
