184 J. B. Johnston 
those few very coarse fibers which in Lampreta enter the acusticum 
and run far caudally in it. As these fibers are to be traced only 
in GOLGI sections their presence in the present species could not be 
determined. Compare Chrmaera (6,4). 
The trigeminus has two small dorsal branches from its ganglion, 
supplying the dorsal surface of the head, and probably the ramus 
ophthalmieus superficialis V, which in Petromyzon is combined with 
the profundus, is to be regarded as another dorsal branch of the 
trigeminus. The rami maxillaris and mandibularis present three 
important peculiarities as compared with those of other fishes: the 
separation of the part which seems to correspond to the mandibu- 
laris into a pure motor and a mixed motor and cutaneous ramus; 
the presence of motor fibers in the maxillaris; and the distribution 
of a part of the maxillaris to the floor of the mouth. "The pure motor 
ramus requires no comment except to say that its course is due, a8 
are so many peculiarities in the head of Petromyzon, to the great 
extent of the buccal apparatus. The mixed ramus is interesting from 
the disposition of its cutaneous component. It supplies the skin of - 
the lateral surface at least as far caudally as the position of the 
spiracular cleft. Just where this cleft was situated can not be 
ascertained in ammocoetes of this age, but it must have been at or 
cephalad to the front end of the groove which eonnects the gill slits. 
As the nerve is readily traced to the front end of this groove I think 
it probable that it innervates the skin for an appreeiable distance 
behind the spiracle. Since the opereulum in ganoids and teleosts is 
developed from the outer part of the hyoid segment, it will be seen 
that the area innervated by the terminal branches of this nerve 
eorresponds to the outer or cephalie surface of the opereulum. The 
skin of the opereulum is regularly innervated in part by a branch 
of the trigeminus and I believe that the eutaneous component of this 
mixed ramus in Petromyzon correspondends to that opereular ramus. 
If this be correet, the eutaneous component of the mandibularis 
which in other forms innervates the lower jaw is not represented in 
the two rami above discussed. It is probably to be found, so far as 
it exists in Petromyzon, in that branch of the maxillaris which 
innervates the floor of the mouth and the tongue. This branch can not 
represent the part of the mandibularis which innervates the outer skin 
of the lower jaw, and it seems probable that that portion is almost 
or quite lacking in Petromyzon. The reason for this must also be 
looked for in the relations of the buceal apparatus. The mandibular 
