420 ON PTERASPIS DUNENSIS 
of these tissues in any Cephalopod or Crustacean with which I am 
acquainted—the construction of the cuttle-bone being totally different ; 
and they exist, in combination, in no animal structure which has yet 
been described, except Pterasps. In form, and in the presence of the 
diverging ridges described by Professor Roemer, the fossil perfectly 
agrees with many of our English Preraspides; and I have therefore 
no hesitation in expressing the opinion that Arch@oteuthis must dis- 
appear from the list of Dibranchiate Cephalopods, and consequently 
that the paleontological history of this group cannot, at present, be 
traced back further than the beginning of the Mesozoic epoch. 
The distinction of species among the Preraspides is a difficult 
matter; and, pending investigations which I have been for a long 
time making on this subject, I leave open the question whether 
Professor Roemer’s specimens are or are not types of a new species, 
which, in the latter case, must be termed Pteraspis Dunensis. 
In conclusion, I may remark that, as I have already pointed out 
elsewhere (British Association Reports, 1858), the test of Pteraspis, 
as commonly met with, consists of only a part of the cephalic shield 
of that singular fish, the whole shield being not a little similar to 
that of Cephalaspis. In Pteraspis rostratus, for example, the entire 
shield has the form indicated by the subjoined outlines, of which A 
represents a dorsal, and B a lateral view. It consists of a cephalic 
rostrum (a), more or less elongated and pointed according to the 
species, passing posteriorly into the broad shield (4), which (as the 
dotted lines indicate) is commonly found broken off and alone. 
When perfect, this is produced laterally and posteriorly into two 
cornua (¢), and in the middle line behind passes into a broad pro- 
longation (a), which gives rise interiorly to a long, curved, and back- 
wardly produced spine (2). Upon each side of the test, where the 
rostrum joins the rest of the shield, there is a round, well-defined 
aperture (/), which may be either the orbit or the nasal aperture. 
It is not easy to find an exact parallel for such a cephalic covering 
as this among existing fishes. Lorecaria, Tetrodon naritus, Acipenser, 
and Spatilaria seem to present the nearest analogies—the two former 
being much more remote than the two latter. In fact, if the bony 
cephalic shield of the Acipenseroid fishes were ossified in one piece, 
it would very closely resemble that of both Cephalaspzs and Preraspis, 
and would hardly differ more from either than the two from one 
another. 
