ORTMANN: A MONOGRAPH OF THE NAJADES OF PENNSYLVANIA 299 
is noticed only in the eggs, while later on the glochidia are more or less free. In 
Quadrula coccinea and Pleurobema, however, the placentze are more solid, and remain 
so until they are discharged (See below). 
In Strophitus, as we have mentioned, peculiar conditions prevail. The ovisac 
(inner part of a water-tube) is subdivided into smaller partitions, running cross- 
wise to the gill; and in each of these compartments, which are almost cylindrical, 
a small number of ova are located which again stick together rather firmly 
and can easily be taken out whole. These “placente” of Sterki, consequently, 
are not entirely homologous to the placentz of the other genera, since in the latter 
each ovisac contains only one placenta, while in Strophitus there are many of them 
in one ovisac. The eggs of Strophitus form a single, irregular row in each placentula, 
as I should prefer to call them here, and there are from about two to eight or ten 
(generally seven) in each placentula.° 
In the genera Anodonta, Anodontoides, Symphynota, and Alasmidonta, no pla- 
centee appear to be present. The eggs as well as the glochidia fill the ovisacs in 
immense numbers, and seem to be entirely free. At any rate I never succeeded in 
isolating the egg-mass from an ovisac, but invariably, as soon as an ovisac is 
injured, the eggs or glochidia flow out freely, without sticking together, although 
in some instances in some of my sections of the genus Symphynota, a placenta-like 
cohesion is here and there indicated in the younger eggs. 
2. The Glochidia. 
I am not prepared to say where the fertilization of the eggs takes place. It 
must take place somewhere between their issue from the genital orifice, and their 
final deposition in the gills. It may take place in the suprabranchial canals, or 
in the gills. At any rate as soon as the marsupium is well filled cleavage begins, 
and the eggs develop within the gill into the young larval shell, known as glochidium. 
The development from the egg to the glochidium is rather rapid, while the glochi- 
dium may remain unchanged a long time in the marsupium before it is discharged. 
We generally find in individual shells a rather uniform stage of development 
of the embryos, and, as a rule, when there are eggs, there are only eggs, and when 
there are glochidia, we find only these. Nevertheless, as already observed by 
Sterki (1898, p. 19) in Cyprogenia, there are sometimes cases, where all stages from 
the egg to the fully developed glochidium are found in one and the same marsupium. 
SI have evidence in some of my slides that not all eggs in a placentula of Strophitus develop into glochidia, but that 
Some become abortive. It may possibly be that the latter furnish material for the placentule. In other species also I 
have seen what looks like abortive eggs (see Lampsilis alata, Pl. LXXXIX, fig. 18). This matter should be investigated 
more closely. Sterki (1898, p. 19) already mentions it. 
