68 
THE GEOLOGIST. 
have been saved, and are now in my possession. Altliougli the matrix in 
which they have been preserved is so ver^^ coarse that the various layers 
composing the cephahc shield are not sufficiently preserved to show their 
characteristic structure, and hence Professor Huxley failed to identify my 
specimens with any of the named English species, yet its form is very per- 
fectly exhibited ; and in the cast, the spine which proceeded from the 
posterior part of the head is well shown. The rough sketch I herewith give 
is a tolerably correct representation, and is of 
the size of nature ; from this it will be seen 
that it much more nearly coincides with Pro- 
fessor Huxle5''s restoration than with that of 
Mr. Mitcliell. The head-plate had evidently 
been fonv-ed of two parts, the anterior resem- 
bling in shape the head of a small Cephalaspis, 
but rather more elongated ; the posterior por- 
tion is hj much the larger of the two, its 
length being nearly two and a half times 
that of the anterior ; its shape is nearly oval, 
truncated behind, with a short cusp or horn 
On each side, and in the centre stretching 
backwards and ujDwards, terminating in a 
sharply pointed spike or spine. A very dis- 
tinctly incurved ridge, but of no great eleva- 
tion, commences about halfway back on the 
posterior plate, and terminates in the above- 
mentioned spine. There are indications at 
the edges of the head, v^ery close to the junc- 
tion of the two plates, of what may have been 
the eye orbits, but these are indistinct. The 
principal if not the only points of divergence 
betwixt the specimen and Professor Huxley's 
restoration are, the position of the lateral ter- 
minal cusps, and the absence of any division 
betwixt these and the head-plate, and in the 
form and size of the posterior elongation (nu- 
chal spine), which in my specimen seems to be a well-formed, round, 
sharply pointed spine. 
Mr. Lankester, in comparing Mr. Mitchell's restoration with that of 
Professor Huxley, does not, in my opinion, sufficiently allow for what may 
have been specific differences of form ; but at the same time, unless Mr. 
Mitchell finds what he figures as the prolonged central termination, in such a 
position as to afford undoubted evidence of its forming part of the same 
head, its size seems to me so sadly exaggerated that I cannot but regard it 
as having formed part of some other, probably very different creature. 
What renders this the more likely is, that I have examined many frag- 
ments which, although too imperfect to found on for any new genus, seem 
to belong to some nearly allied form, and are evidently parts of neither 
Cephalaspis nor Pteraspis. I am also much inchned to suspect that Mr. 
Mitchell's third figure has been built up of such fragments. Several 
heads of Cephalaspis which have been in my hands, go far to show that 
the larger part of the under portion of this fish's head had been covered 
by integument ; and judging from the many points in which Cephalaspis 
resembles the Sturgeon of our own seas, I have little doubt but that, like 
it, it was furnished with a sucking apparatus for its mouth : and in all 
probability the under portion of the head of Pteraspis was similarly 
formed. 
