490 DAVIDSON BLACK 
of Hexanchus both glossopharyngeal and abducens fibers may 
be cut in the same transverse section (fig. 13). 
Motor trigeminal nucleus and root (Nu. et. rad. mot. N. V.). 
The motor trigeminal nucleus in Selache is placed in a dorsal 
position on the level of its root exit, though it extends some dis- 
tance both rostrad and caudad of this plane. This situation is a 
characteristic one for this nucleus in sharks (fig. 17). 
In the reconstruction chart figure 17 C, the motor root of the 
trigeminus in Selache appears to be very small when compared 
to that of Hexanchus or of Heptanchus. This results from the 
CO Dexia. 
Fig. 13 Hexanchus. Outline of transverse section through the medulla to 
show the emergence of abducens and motor glossopharyngeal rootlets at the 
same level. Abbreviations: N.V/., abducens root; R.[X.m., motor rootlet of 
glossopharyngeus. Other abbreviations as in figures 10 to 12. 
fact that the root emerges from the medulla in Selache in a very 
compact bundle, so that its superficial attachment extends over 
fewer sections than on the other forms. 
The position of the motor V nucleus in transverse section is 
indicated in figure 14. Here it will be seen that the cells show a 
distinct tendency to develop processes chiefly in a ventro-lateral 
direction and the nucleus as a whole is somewhat more ventro- 
laterally placed than that of the motor VII (fig. 11). In this 
position the motor V nucleus is placed in close relation to the 
reticular grey surrounding the upper end of the descending 
sensory V, and the inconspicuous secondary gustatory tract. 
