144 PALEOZOIC FISHES OF NORTH AMERICA. 
It is as distinctly marked in the head of Titanichthys, and its position is indi- 
cated in the figure of the cranium of 7. Agassizii given on Pl. I and copied 
from a photograph. I have also represented the inside of the ethmoidal 
plate of Dinichthys with the funnel-shaped orifice penetrating it on Pl. LI, 
Fig. 3. It seems to be homologous with the aperture in the cranium of 
Reptiles, Amphibians, and the Siluroid fishes which has been called the 
pineal eye, and about which so much has recently been written. It has 
long seemed to me that the functions of this aperture, which is so very 
marked in the Siluroids, should be investigated by the microscopic study 
of the soft parts, and at my suggestion Mr. Bashford Dean, one of my stu- 
dents, has begun a research which it is to be hoped will clear up this ques- 
tion so far as the fishes are concerned. 
A color of probability is given to the suggestion that this orifice was at 
one time an eye, by the fact that in Pterichthys and Bothriolepis a dumb- 
bell or double aperture on the top of the head formed almost certainly the 
organ of vision; no other eyes being known in these fishes and no other 
use for these apertures being conceivable. In the allied but very distinct 
Cephalaspis the eyes are separate but closely approximated, as though the 
ancestors of this group of fishes had been monoculous; Pterichthys and 
Cephalaspis representing different stages in a progressive separation of the 
sight organs. If this were true, however, Dimichthys, which was certainly 
as old as either, had progressed very much further along this line of devel- 
opment; since the eyes in this genus were as fully elaborated and special- 
ized as in any living fishes. 
It is certain that the perforation of the cranium in Dimichthys and its 
allies, as well as in the buckler-headed Siluroids, in which the brain-box is 
so strong and complete, served an important purpose, and it is to be hoped 
that the investigations of Mr. Dean will tell us what was its function. 
Tue Fins or Drnicutuys. 
Plate VII, Figs. 1-1". 
Ossified fin-rays have several times been found by Mr. Terrell asso- 
ciated with the bones and plates of Dinichthys Terrelli. Usually they are 
separated, but in some cases they have been found lying side by side in 
