1884] in the Silurian Rocks of North America. 1223 
Cephalaspis, which has not yet been reported from the former 
locality. The two species known are: Cephalaspis dawsoni, 
Cephalaspis campbelltonensts. 
During my recent work on the palaontology of Pennsylvania 
I found some fossils much resembling the well-known specimens 
from the “ Ludlow beds” of England, which for fifty years have 
been the oldest known fishes. They -were an Onchus and two 
kinds of shields very close in outward form to the Scaphaspis of 
kester. 
The study of these fossils has proved that this resemblance is 
not merely superficial, and that there unquestionably existed, 
both in Pennsylvania and in England, similar species of fish 
which characterized the Upper Silurian era in both continents. 
The shield, like that of Scaphaspis, as described and figured 
by Huxley (Q. J. G. S., Aug., 1858), consists of three layers. The 
inner layer is thin and homogeneous, having a thickness of about 
: ay Of an inch. On this reposes a cellular layer about sy of 
ri 
: 
y 
‘ 
3 
i 
i 
4 above them. 
: 'S composed of the same material as the two alrea 
Mtis traversed from end to end by two systems O 
A 
firrows) a little above them and rather less in size, 
| Ü er until they terminate at the oute 
3 ter the microscope they stand out distinc 
: branches communicate with another sy 
ted from its upper surface, is a delicate and 
an inch in thickness, the walls of which are composed of the 
same material. These cells open on the interior surface by round 
_ Pores which traverse their floor, that is, which pass through the 
inferior layer of the shield. The cells also communicate by 
Openings in their covering layer with a system of canals lying 
of an inch, and 
dy described. 
f vessels of 
Which the lower is immediately above the cellular layer, with 
which it communicates by the openings above mentioned. The 
“nals of this system have a diameter of about zł of an inch, 
and from their sides issue branches almost as large as themselves. 
stem of canals (or 
running paral- 
he superficial 
The outer layer has a thickness of about 170 
€l with them from end to end of the shield under t 
Striation, 
ms of canals, and 
beautiful thicket 
tute branching tubules, becoming continually smaller and 
r surface of the shield. 
tly in black on the ; 
composes the lay- 
Connected with the former of the two syste 
nt background of the material which 
