‘ 
298 MEMOIRS OF THE CARNEGIE MUSEUM 
From this point the way is open for the eggs all along the base of the inner gill 
backward to the common cloacal chamber and forward again into the suprabranchial 
canal of the outer gill. From the suprabranchial canals at the bases of the gills 
the ova reach the water-tubes in the gills. I have repeatedly found specimens, in 
which this process was going on, and where both suprabranchial canals were more 
or less filled with ova. (Instances are Ptychobranchus phaseolus, Strophitus eden- 
tulus, Alasmidonta undulata, Quadrula undulata.) 
After the eggs have reached the water-tubes (ovisacs), they are firmly packed 
together there, until the marsupium is filled to its utmost capacity. In no 
case have I observed that the ova become connected in any way with the tissue 
of the gills; on the contrary, they always remain free from them, but generally are 
connected with each other, so that the contents of each water-tube or ovisac form 
a more or less solid mass, which in its shape conforms to the shape of the ovisac. 
The connection of the eggs between themselves is merely a “sticking together,” 
which sometimes is rather slight, but produces in other cases a rather firm mutual 
adhesion. It is accomplished by the outer egg-membranes, which apparently have 
the property of freely imbibing water, and thus of becoming gelatinous.’ For 
these gelatinous masses containing the eggs we may use the term placenta, intro- 
duced first by Sterki with reference to the genus Strophitus. (As will be seen later, 
I call the placente of Strophitus placentule.) ‘These placentee may be more or 
less compact (See Pl. LXX XVI, fig. 3; Pl. LXXXVIL, fig. 3; Pl. LXXXVIII, fig. 
14); they are well developed in Quadrula subrotunda, kirtlandiana, and rubiginosa, 
where they are rather cylindrical, since the water-tubes do not expand very much 
and remain subcylindrical in the charged marsupium. In Ptychobranchus the 
placentz are also very solid, and there is in the outer layer a peculiar brown stain 
developed, when the glochidia are formed. The placente are subcylindrical here, 
but somewhat club-shaped, on account of the swollen distal end. In all these 
forms, it is easy to take the placentz out of the gill whole, without injury. 
In Quadrula metanevra, undulata, coccinea, in the species of Plewrobema and 
Unio, in all forms of the Lampsilis-type (except Ptychobranchus), the water-tubes 
are more or less stretched out transversely, so that their lumen becomes compressed, 
and consequently also the placenta have a similar shape, leaf-like, lanceolate, or 
elliptical. Here the mutual adhesion of the eggs is not so firm, so that they come 
apart more easily, and it is not feasible to take the placente out entire. In some 
of these forms (Quadrula undulata, Unio, and the Lampsilis-species), the adhesion 
’There may be a secretion furnished by the epithelium of the water-tubes. This question is reserved for further his- 
tological investigation. 
