66 
UEOKGE E. NICHOLES. 
appears to be between 2 /x and 3 /x, altliongh in several 
instances it was found with a thickness of 4’5/x, even where 
there appeared to have been bnt slight retraction. 
Development. 
Of the larval stages of Petromyzon 1 have had no material 
beyond the single series of transverse sections through the 
head of an ammococte 57 mm. long, this being the specimen 
in which the “ ciliated grooves ” of the sub-commissnial 
organ were described by Dendy (^02). 
In this series lieissuer’s fibre cannot be identified with 
certainty, nor could the large cells, said by Sargent to give 
rise to the fibre, be distinguished, although that author stated 
that he was able to discern these cells in ammocoetes of P. 
planeri from 6 mm. to 10 mm. in length. 
I have, however, been able to prepare and examine a faiily 
complete series of sections through the whole or parts of 
larva3 of Ichthyomyzou (E n t osp h e n u s) tridentatus, 
varying in length from 12 mm. to 105 mm , and while, so far 
as I could find, tlie ammocoete of this genus does not differ 
markedly from that of Petromyzon, my findings are, never- 
theless, markedly unlike those of Sargent, who examined 
P . planeri. 
As, so far as I know, the nervous system of the ammocoete 
of this lamprey has never hitherto been described, I propose 
to give a short account of the condition of the roof of the 
brain at certain stages in development, dealing simply with 
those parts of the brain with which this research is chiefly 
concerned. 
Ichthyomyzou (En tosphen us) tridentatus. 
fl'his lamprey inhabits the North Pacific Ocean, and is said 
to take the place in those wateis of the very simitar lamprey 
of the Noihh Atlantic Ocean, Petromyzon marinus. Like 
P. marinus, it grows to a large size, and although the adult 
is not, I believe, easily obtainable, the ammocoetes are found 
