82 ALUS. [Vol. XVIII. 



to inheritance, or to oth^r causes than a simple invasion of the 

 obliqui muscles of the adult animal. 



The olfactory nerve, after it leaves the anterior olfactory ex- 

 tension of the cranial cavity, traverses the anterior part of the 

 orbit and so reaches the olfactory canal through the antorbital 

 process, which it traverses accompanied by two veins. In Amia 

 the olfactory nerve does not so enter the orbit. In that fish it 

 simply runs across a relatively large opening that lies ventro- 

 lateral to the nerve and opens directly from the long olfactory 

 canal of the fish into the extreme anterior end of the orbit (No. 4, 

 p. 513). This opening in Amia gives passage to a vein coming 

 from the nasal pit, and was considered by me as the homologue 

 of the posterior opening of the orbito-nasal canal of Selachians. 

 I accordingly called it the orbito-nasal opening. Immediately 

 ventral to it, in Amia, the obliqui muscles have their origin, arising 

 in a slight depression in the cartilage of the chondrocranium. 



The orbito-nasal opening of Amia is thus seen to be entirely 

 wanting as a distinct and separate opening in Scomber. It may, 

 in this latter fish, be simply fused with the posterior opening of the 

 olfactory canal through the antorbital process, or, what seems 

 much more probable, it may have become so greatly enlarged in 

 Scomber, by the deepening of the orbit, that it has entirely dis- 

 appeared as a separate feature of the skull. If this be so, the 

 slight ridge that is always found on the orbital face of the pre- 

 orbital ossification, close to the ventro-lateral edge of the pos- 

 terior opening of the olfactory canal, may be a persisting part 

 of the edge of the former opening; and as the lower end of this 

 ridge lies approximately opposite the lower edge of the surface 

 of origin of the obliqui muscles, the posterior outline of that sur- 

 face may represent the former continuation of the edge of the 

 opening. The olfactory nerve of Scomber would, in that case, 

 simply run across the opening, as it does in Amia, the great en- 

 largement of the opening and the concomitant narrowing of the 

 interorbital septum leaving it free in the orbit. 



The Petrosal {PE) is an irregular bone of somewhat variable 

 shape. Its outline, as seen from the outer surface of the skull, 

 is not at all that of the thick body of the bone, thin plates of bone, 

 with serrated edges, projecting in several directions from its super- 



