178 J. B. Johnston 
at their origin and in front of the rectus posterior. It continues 
forward in this position, elosely applied to the rectus superior, until 
the front edge of the reetus inferior is reached, when it inelines 
laterad and goes forward mesial to the buccal nerve and enters tbe 
museuli obliquus anterior and rectus anterior. The III. nerve clearly 
innervates the two muscles last named, and as it is for some di- 
stance practically imbedded in the rectus superior and reetus in- 
ferior, t0 which no other nerve is to be traced, I think it highly 
probable that it innervates these also. These four museles are 
derived (KOLTZOFF) from the first somite and are homologous with 
the muscles innervated by the oculomotorius in other vertebrates. 
The IV. nerve arises in the usual manner and passes somewhat 
backward to meet the profundus root just as it enters the eranium. 
The trochlearis passes throug the foramen elose upon the dorsal sur- 
face of the profundus, continues laterad over the dorsal surface of 
the profundus ganglion, goes outward and downward and enters 
the obliquus posterior muscle. This agrees with the description of 
P. FÜRBRINGER and KOLTZOFF, except that in the present species 
the trochlearis runs over the profundus instead of under it as in 
P. marinus and planeri. The m. obl. posterior is the homologue of 
the m. obl. superior. 
The case of the abducens is much more diffieult. In this species 
of Petromyzon, as in Lampetra, there is not the slightest trace of 
a true abducens root having its origin from the ventral motor column 
and its exit from the ventral surface of the brain, either in the region 
of the VII" or elsewhere. There is in Lampetra, however, a com- 
ponent in the motor root of the trigeminus which arises from an 
isolated portion of the ventral motor column at the level of the 
VII—VIII nerves. In P. marinus, fluviatilis, planeri and Lampetra 
there are two motor roots adjacent to the sensory trigeminus, the 
smaller of which has universally been called the abducens. The 
writer has questioned the correetness of this, since in Zampetra the 
root is much larger than is necessary for the innervation of one or 
two eye muscles, and more especially since it arises from the lateral 
motor column and is therefore a visceral motor nerve. In the Ammo- 
coetes here described there is a separate slip of the motor trigeminus 
which arises a little dorsal to the main root and thus corresponds 
to the so-called abducens. It is readly traced centrally to a compact 
group of cells in the dorsal part of the lateral motor nucleus, whose 
fibers are clearly seen going into this strand. T'he motor trigeminus 
