33 2 
The Ohio Naturalist. 
[Vol. XI, No. 6, 
It is known that the ova, from the ovary, pass through an 
oviduct on each side into the branchiae, where they develop into 
embryos, the so-called glochidia. The glochidium, of about the 
size of the ovum, has a two-valved shell, very different from the 
postembryonal shell, and also of markedly different formation in 
the several groups, and a very primitive formation of the soft 
parts, without alimentary canal, ganglia, branchiae, etc. 
The formation of the female reproductive branchiae is varied 
and furnishes principal characters for classification. In some of 
the groups, the Unioninae ( Unit >, Pleurobema, Quadrula), also the 
Anodontinae ( Anodonta , Alasmidonta , Gymphynota, etc.), the 
branchiae which receive the ova, in their whole extent, show 
only slight and macroseopically barely noticeable differences’ from 
the male branchiae, and the non-receptive of the female. In a 
still higher group, only a part of each of the outer branchiae is 
noticeably differentiated, the so-called marsupium, consisting of 
ovisacs, their number being very different in the several groups, 
and approximately constant in adult individuals of each species. 
Also their configuration shows differences, when barren, and much 
more so when charged. This is the group, or subfamily Lamp - 
silinae, and, with some differences, Proptera. In Ptychobranchus 
(e. g. phaseolus Hildreth), the outer branchiae are differentiated in 
their whole extent, and of a formation markedly different from 
that of the others, when gravid. 
In the lower forms, there are no or slightly marked differences 
of the shells between males and females. With the appearance of 
the marsupium which, when filled and distended, projects more 
or less over the general contour and the lower edge of the branchiae, 
there comes a corresponding distension of the shell in the female, 
not or slightly marked in some forms, strongly so in others, e. g., 
most of the species of Lampsilis. It reaches its highest grade in 
Truncilla, where that part of the female shell is not only greatly 
distended but also of a formation and sculpture different from 
the rest of the mussel. 
These differences, gradations, of both soft parts and shell, are 
naturally not in a straight line, the same as in other groups of 
animals, but with ramifications and gaps, which latter would 
probably be bridged over by extinct forms, and possibly by such 
as are living in other zoo-geographical provinces. 
In connection with the different formation of the gravid 
branchiae, there are also different ways of discharging the embryos. 
In the Unioninae the young are expelled upward from the brood 
chambers into the suprabranchial canal and from there out into 
the water through the anal siphonal mantle opening. But in the 
Lampsilinae, each ovisac opens, at its inferior end, and 
the contents, coherent as a cake (“placenta”), makes its exit 
through that rent, and out either through the branchial siphonal 
