STRUCTUEE xVND DEVELOPBIENT OE EEISSNEIt’s FIBRE. 67 
plentifully during the early summer in certain of the rivers 
Avhich draiir the western slopes of the Rocky Mountains, and 
which seem to form the breeding-grounds of the species. 
My specimens, which were collected for me by Mr. W. F. 
Allen from the Carmel R., California, were preserved entire 
in aceto-bichromate fluid. 
They are of all sizes from 12 mm. to 105 mm. The smallest 
specimens, I understand from Mr. Allen, were from two to 
three weeks’ old, so that they have apparently a more rapid 
growth than the ammococtes of P. plaueri, which were, 
according to Sargent, from G mm. to 10 mm. long at from 
twenty-six to thirty days after hatching. This difference is 
probably simply correlated with the considerable disparity in 
size of the adult animals of the two species and must not be 
lost sight of, for, as I do not know the ages of my various 
specimens, I shall be compelled to designate them by their 
respective lengths, which will probably be much in excess of 
those of ammocoetes of l iver or lake lampreys of the same age 
and degree of development. 
My smallest specimens included one of 12 mm. and another 
of 14 mm. which were cut transversely, and others 13 mm. 
(two) and 14'5 mm. which were cut sagittally. 
In all of these the roof of the mid- and hind-braiu is still 
almost entirely epithelial. The ]>lica r horn bo- m esen - 
cephalica is, however, already well marked, although still 
oidy one layer of cells in thickness. At its anterior end the 
membranous I’oof of the mid-bi'ain (llie future tela choroi d ea 
II) passes into the posterior commissure, which, even in my 
youngest sjtecimeus, is quite well defined. Although this has 
not, as yet, become downfolded to form the plica nieso- 
proseucephalica, its hinder end has already come to lie at a 
slightly lower level than the developing choroid plexus (see 
'J’ext-fig. 6). 
In the fore-brain the greater part of the roof is occupied 
by the habenular ganglia, of which the right is enonuously 
the larger. Above these lie the two epiphysial outgrowths, 
that of the right side being lai’ge, and having apparently. 
