The Cranial Nerve Components of Petromyzon. 197 
in its definitive position the cutaneous fibers secondarily aequired 
the shorter course by way of the branchial nerves. In the case of 
the last branchial nerve this has not happened. 
Distinguishing Features of the Species P. dorsatus. 
The study of the ceranial nerves of this species shows so many 
and significant peculiarities that one can scarcely doubt the justice 
of Professor GAGE’s conclusion that it deserves rank as a separate 
species. Here important anatomical characters are present eoineidently 
with the independence in breeding upon which Professor GAGE bases 
his conelusion. The most important of these characters is perhaps 
the presence of two ventral motor roots in the oceipital region not 
found in ammocoetes of other species of Petromyzon. Next in impor- 
tance is the peculiar disposition of the neuromast components in the 
VII—X anastomosis and in the IX!" and X" nerves. Other diffe- 
rences are found in the number and arrangement of the roots of the 
IX th and X" nerves and their ganglia. None of these characters 
can be used for determining the species of specimens when collected, 
but I venture to suggest that the differences in the number and 
position of the pit organs in Lampetra and P. dorsatus may serve 
that purpose. In the specimens thus far examined by me there are 
a larger number of organs in both the supra-orbital and infra-orbital 
rows in P. dorsatus and a smaller number of organs near the mid- 
dorsal line of the head in the same species. In Zampetra the hyo- 
. mandibular organs form a more definite row and extend farther 
forward on the upper lip. If upon examination of a large number 
of animals, suffieient differences of this kind be found constant between 
adults and between larvae, they could be used for distinguishing the 
Ammocoetes of the two species. 
General Results. 
In the arrangement of the cranial nerve components Petromyzon 
agrees in general with the fishes, but shows some marked primitive 
characters. 
The profundus nerve retains its independence but has added to 
it the equivalent of the rami ophthalmiei superfieiales trigemini and 
facialis of fishes. 
The trigeminus encroaches upon the general eutaneous area of the 
hyoid segment in the dorsal region. The ramus maxillaris includes 
a part of the ramus mandibularis of fishes. 
Morpholog. Jahrbuch, 34. 14 
