COEEESPONDENCE. 
171 
and finished lateral outline of the posterior head-plate, I think it extremely 
improbable that this species ever possessed the lateral small plates forming 
the cornua of Pteraspis rostratus ; certainly none of the many fragments in 
our collections show any vestige of these. One of my specimens has the 
occipital spine in situ, and in several of ]Mr. Mitchell's the spine is shown 
detached; thus differing from the occipital crest of Cephalaspis, which 
forms an integral portion of the head plate. This spine is short, stout, 
striated longitudinally, and is deeply inserted in the substance of the head 
plate, in which it seems to have been inmovably fixed. In their composi- 
tion the head-plates are quite similar to that of the English species, some 
of Mr. Mitchell's specimens having, as noticed by him, the exterior stria- 
tion, the internal reticulated markings, and the inner nacreous plates or 
lamellae well preserved. From his specimens I have little doubt that the 
perforations at h, in the figure given in your number for March last, are 
indeed the eye orbits ; while those at a are too distinctly marked to have 
been the result of accidental fracture, whatever may have been their nature. 
As drawn in Mr. Mitchell's latter restoration, and in my figure (Geol. 
Feb. 1863), the test consists of only two distinct plates, an anterior and 
posterior, with a distinct spine. 
No light has yet been thrown on the nature of the under surface of the 
head, some of our many fragments may possibly belong to this part of its 
body ; to me, however, they all seem mere broken fragments of the upper 
cephalic plates. 
As to the oral appendages, until very recently I was of opinion that 
these were of the nature indicated in Mr. Lankester's letter, both in this 
genus and in Cephalaspis. This opinion was founded not only on negative 
evidence, but ulso on the form of tlie plate protecting the under surface of 
the head of the latter genus, and in my letter referred to (Feb. 1863) I 
expressed this conviction pretty strongly. During the course of last au- 
tumn, however, I had the good fortune to open out some magnificent spe- 
cimens of Cephalaspis LytUi, in which the position and character of the 
mouth and teeth are distinctly exhil)ited. The mouth opened immediately 
under the cephalic plate, the gape occupying about one-thh-d of the entire 
outer margin, the upper maxilla\ or jaws, anchylosed with the cephalic 
])late, forming an integral part of it, and are finished with a single row of 
short, stout, slightly flattened teeth, which extend quite round to the cornua 
or cusps. In one of my specimens, a portion of the lower jaw is preserved 
with its single row of similar teeth. From the decided analogy between 
Cephalaspis and Pteraspis, it is probable that the latter had been similarly 
provided. One of Mr. Mitchell's specimens seems to bear this out, havmg, 
as noticed by him, the anterior margin of the anterior plate turned down- 
wards and inwards, as in all our moderately well preserved heads of Ce- 
phalaspis. The analogy between these genera is further confirmed by Mr. 
Lankester's most interesting discovery of the scales of Pteraspis, stated 
by him to be similar to the dorsal series of Cephalaspis,— meaning, I pre- 
sume, the bony rings covering the body of this creature. These, however, 
my specimens show to have been again covered externally by scales similar 
to those covering the cephalic plate. 
It seems to me that as yet the nature of the CephalaspidfB is very imper- 
fectly understood. I strongly suspect the cephalic plate to consist ot the 
various cranial bones anchylosed, while the bony rings protecting the body 
equally appear to represent the vertebra? and ribs ; all covered externally 
with scales, or rather dermal scutes, thus indicating that this family may 
have held among the fishes a place somewhat, although by no means ex- 
