THE OLD RED SANDSTONE EISHES OE ENGLAND. 445 
regard it as merely a species of Cephalaspis ; but since so 
eminent an authority as Sir Philip de M. Grey Egerton was the 
founder of the genus, we cannot dispute the propriety of the 
separation. We know nothing of the body of this little fish, 
although we may be pretty certain that it was similar to that 
of Cephalaspis. Its remains are rather plentiful in the passage 
beds which connect the Old Bed Sandstones with the Silurian 
rocks ; from Ledbury in Herefordshire we have seen a specimen 
of rock with as many as a dozen heads upon it. 
The last genus which we have to describe is the Pteraspis, 
and of the three it is, perhaps, the most interesting ; for many 
mistakes have been committed in assigning the fossil remains 
of this fish to various classes of creatures, and it is only quite 
recently that we have had undeniable proof of their piscine 
nature. Many years since, Agassiz received fragmentary 
specimens of the cephalic shield of the Pteraspis, and he at 
once concluded that they belonged to a fish which he placed 
in his genus Cephalaspis. Since then Dr. Kner of Vienna 
obtained some imperfect specimens^ and remarked that their 
elongated oval form was very similar to that of the cuttle-bone ; 
and on making a perpendicular section through the shell or 
test he found a structure which he considered to be very 
similar to that shown by the “ pen ” from the interior of the 
Sepia officinalis , or common cuttle-fish. Dr. Kner therefore 
placed these remains in a new genus, Pteraspis (7rr£pov, a 
wing — acnr'iQ, a shield), and classed them among the mollusca. 
The matter, however, was not allowed to rest thus ; the truth 
was still to be discovered. Professor Huxley made a careful 
examination of the microscopical structure of the cephalic discs 
of both Cephalaspis and Pteraspis, and having better specimens 
than Dr. Kner had at his command, came to the conclusion that 
Pteraspis was a fish, but generically distinct from Cephalaspis. 
In fig. 10 we have drawn a highly magnified view of a perpen- 
dicular section through the cephalic disc of Pteraspis, and in 
fig. 11 a less highly magnified view of the surface is given. 
The test consists of three layers of bony matter deposited in 
layers ; the outermost one is striated, or delicately grooved, 
like the human skin on the palm of the hand. The middle 
layer is composed of polygonal cavities, and the internal 
layer, which is the thickest, is made up of a nacreous la- 
minated substance. These layers may be seen in the figures. 
Canals ramify through these layers, and end in an arborescent 
form in the external layer. There are no lacunas, or bone 
cells, as in Cephalaspis, and this affords an important dis- 
tinction between the two fish. But the general contour of 
the cephalic disc of Pteraspis differs considerably from that of 
Cephalaspis, although the type of construction is the same 
vol. hi. — NO. XII. 2 II 
