90 THE NAUTILUS. 
In cornutus Bar., the sacks are also permanently differentiated, 
about six or seven on each side, near the middle of the outer 
branchiz, and considerably projecting over their edges, much as in 
irroratus. But while the latter were found gravid in fall; the few 
cornutus seen, had the marsupia empty at that time, (late in Octo- 
ber); the ovaries were filled with ova and the testes with sperma. 
More observations are necessary. 
U. phaseolus is so different from all other species and groups that 
Simpson and Wetherby are certainly right in regarding it as the 
representative of a distinct genus. The outer branchie, in their 
whole extent, are permanently differentiated, much less high than 
the inner ones, and with a brownish edge. Thin and even while 
barren, they are much larger when gravid, and heavily, somewhat 
irregularly plaited, the folds being caused by the considerable increase 
of the length of the branchize. The sacks are very numerous; in a 
large specimen, 283 were counted on one side. Each one is formed 
by a thin, translucent, yet rather strong and somewhat rigid mem- 
brane, enclosing the ova, or the young, loosely inserted in the sub- 
stance of the branchiz, with a projecting, half-globular head. It 
can easily be extracted, and, when the young are mature, probably 
is expelled in toto. 
The young, although in the shape of the shell not much different 
from other species, shows marked peculiarities of the soft parts. 
The uteri were found charged from July, or August to April, in 
numerous specimens. More exact data must still be obtained. 
Most of the remaining species of the old genus Unio show no 
differentiation of the branchiz or parts of them which are destined 
to lodge the ova and the young animals. The outer branchie, in 
adult specimens, are charged in their whole extent, and often* also 
the inner pair; while gravid, they show only a general bulging, 
but no differentiated or prominent parts, and after the young are 
discharged, they are in no way different from “common” gills, 
except a somewhat ragged margin now and then in old specimens. 
Of a number of species, those cited by Lea, and some others, we 
know that also their inner branchiz were found charged, but we do 
not know whether this is constantly so or not, and whether in all 
species it may be found occasionally. This uncertainty is partly 
‘In the writer’s article, l. c., p. 91 there is a sad, unintentional lapsus, and 
cited by Mr. Simpson (1. ¢. ), about this point ; the correction will be found 
in the above. 
