MOTOR NUCLEI IN PHYLOGENY 509 
caudal end of the nucleus corresponds to the level of the caudal 
border of the oculomotor root (fig. 25). In Polyodon the sub- 
division of the elements of the small oculomotor complex into 
dorsal and ventral cell groups is unmistakable and presents an 
interesting contrast to the conditions obtaining in the large 
undifferentiated oculomotor nuclei in Selache. 
The trochlear nucleus in Polyodon is placed some distance 
caudal to the oculomotor nucleus. Its relation to the fasciculus 
longitudinalis medialis is essentially similar to that of the dorsal 
cell group of the oculomotor complex to the same structure. 
From the trochlear nucleus a very fine strand of fibers arises. 
In its caudal course the root passes dorso-laterally through the 
ventricular gray into the valvula. Immediately beneath the 
floor of the crevice between the valvula and the cerebellum, the 
nerves of either side decussate and emerge from the dorsal 
surface of the brain in the mid-line. The peculiar transverse 
band which the two trochlear roots form at their decussation 
between the valvula and the cerebellum, is shown in figure 22. 
Garman (I. ¢c.) has drawn attention to the similarity of the 
trochlear exit in Acipenser and Polyodon. The nerve roots 
course laterad after their exit and then pass rostrad between the 
auricular lobes of the cerebellum and the tectum (fig. 23). 
In Polyodon, Acipenser and Lepidosteus the position of the 
trochlear nucleus with reference to its root exit and that of the 
oculomotor nerve is almost identical. In Amia, however, in 
correspondence with the more rostral position of the oculomotor 
nucleus, that of the trochlear nerve is placed nearer the level 
of the caudal border of the oculomotor root than in other 
ganoids (fig. 25). 
Discussion 
Spino-occipital complex. In ganoids there are no epibranchial 
spinal muscles as in sharks (97) and the pre-hyal hypobranchial 
elements are represented by the musculus branchio-mandibu- 
laris. Lepidosteus, however, would appear to be an exception 
to the latter part of this statement for according to Edgeworth 
(20) there is no trace of the m. branchio-mandibularis (m. genio- 
