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 iu 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 tegmon 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 tlie model of Stage II., a prominence (Figure A, crt. 

 orb. a.) on the front face of the etinnoid was mentioned. Tliis prom- 

 inence is really a separate cartilaginous mass, resting iu 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 witli the de- 

 scription of the most advanced stage modelled (Figure D). 



d. Stage III b. 



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, whicli I have designated as Stage III b, to distinguish it 

 from the more common condition just described as Stage III a. 



In this specimen (Figs. C and C) 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 i]0 trace of the left supraorbital bar. The right supraorbital {trb. 



