22 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
from the left wing of the ethmoid, though unmaimed individuals whose 
cartilages were otherwise in a like stage of advancement showed no 
traces of it. Furthermore, the stub, instead of disappearing by a grad- 
ual reduction of its diameter in the region midway between the ethmoid 
and the ear-capsule, through which the eye normally passes, preserved 
the bar-like shape — the flat side being directed towards the top of the 
head — until its abrupt disappearance behind the middle region of what 
should have been the path of the migratory eye. Both supraorbitals, 
instead of being backward extensions of the wings of the ethmoid, as in 
most other specimens examined, took their origin from a mes-ethmoid 
enlargement which extended backward directly above the median arch 
that indicates the position of the future interorbital septum. In this 
specimen there was, therefore, a suggestion of a tegmen cranii, such as 
has been described by Parker for the salmon. This, instead of being a 
complete roof, however, was a comparatively narrow plate of cartilage 
which extended backward toward the brain region. 
In describing the model of Stage II., a prominence (Figure A, crt. 
orb. a.) on the front face of the ethmoid was mentioned. This prom- 
inence is really a separate cartilaginous mass, resting in a socket of the 
ethmoid. There is also a pair of small labial cartilages in front of and 
below this plate; but owing to their small size and the difficulty of pre- 
serving small detached processes on the wax plates, they have been 
omitted from the models. In Stage III. this large cartilaginous mass 
has become rounded and projects further forward from the body of the 
ethmoid. Its’ future history will be given in connection with the de- 
scription of the most advanced stage modelled (Figure D). 
d. Stage IIIb. 
The forms of the cartilages change very rapidly at this stage of 
development, and it is with some difficulty that one finds a cranium 
exhibiting a condition intermediate between Stage III a (Fig. B) and 
Stage IV. (Fig. D), which shows the completely twisted head. How- 
ever, I found one fish, larger than many of the recently metamorphosed 
specimens, which I have designated as Stage III 4, to distinguish it 
from the more common condition just described as Stage II] a. 
In this specimen (Figs. C and ©’) the left eye lies in the sagittal 
plane, even though the fish is 15.5 mm. long, the eye usually being 
transformed when the fish reaches a length of 13.5 to 14 mm. There 
is no trace of the left supraorbital bar. The right supraorbital (¢rd 
