508 ALLIS. [Vot. XII. 
of the cavity being filled by fatty tissue (No. 105). This marked 
change in the relative size of the cranial cavity and brain takes 
place mostly in post-larval stages, the unequal development of 
skull and brain being most marked anteriorly. By it the floor 
of the anterior part of the skull, and its perforations, are carried 
forward relatively to the brain, and the optic nerves at their 
origin are, on the contrary, pulled relatively backward ; so that, 
while in the young they issue in front of the canalis transver- 
sus and internal carotid canals, they lie in the adult either 
directly above the internal openings of the carotid canals or 
even behind them. The olfactory branches of the carotids 
must, therefore, first run backward under the optic nerves and 
then forward above them to reach their destination. 
4. Nervus and Lobus Olfactorius. 
The lobus olfactorius, Goronowitsch, or bulbus olfactorius, 
Sagemehl, is in the adult Amia a well-rounded mass (/o/, Figs. 
25 and 64, Pls. XXV and XXXVIIIJ), lying close to the middle 
line of the head, on either side, immediately above the trans- 
verse bar of cartilage marking the front end of the eye-muscle 
canal, immediately in front of the optic chiasma, and imme- 
diately behind the pad of tissue behind and under which the 
nervus opticus enters the orbit. Its diameter is about one and 
one half times that of the nervus olfactorius, and it contains, as 
Goronowitsch has stated, a small lateral extension or diverticu- 
lum of the ventricle of the fore-brain. On its upper surface, 
in well-preserved specimens, there are three slight furrows, one 
running medianward and backward from the lateral surface of 
the lobus, near its anterior end, one laterally and backward 
from the median surface of the lobus, and the third directly 
backward along the median surface, beginning at the point 
where the second furrow begins, near the root of the nervus. 
These three furrows thus correspond fairly well with the 
three furrows shown by Burckhardt on the so-called tuber- 
culum olfactorium of Protopterus (No. 16, Figs. 3 and 4). 
The first two enclose a slightly marked anterior region of 
the lobus. 
