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294 MEMOIRS OF THE CARNEGIE MUSEUM 
(Pl. LXXXVI, fig. 126); in others they were greatly reduced, and the septa had 
assumed their normal shape; but in specimens of this group, where the marsupium 
was well filled with eggs or embryos, I have always seen them, although it is some- 
times hard to obtain good slides, especially when the embryos have reached the 
glochidium-stage, since at that time the making of sections is rather difficult. 
There is one form among those above enumerated, in which the development 
of this structure goes a step further. This is Strophitus. It was known long ago 
that this genus is characterized by a peculiar arrangement of the ova in the gills, 
they forming short, horizontal rows, which run transversely from lamina to lamina. 
According to my investigations this arrangement is brought about by further out- 
growths of the epithelial layers of the septa, which fill the spaces between two septa, 
or rather only the middle part, the ovisac (See Pl. LX XXVIII, figs. 7, 8, 9), and 
thus the simple ovisac of Anodonta and other genera is here divided into a number 
of smaller, secondary ovisacs, running transversely across the gill, each of which 
contains a short, more or less cylindrical mass of eggs or embryos. ‘These masses 
(placentula) will be discussed later. Also in Strophitus these structures are not 
present in sterile females, and after the discharge of the glochidia they soon dis- 
appear. The gradual development is seen in the vertical longitudinal sections of 
the marsupial gill on Pl. LX XXVIII, figs. 6 to 9. 
All other forms of Unionide, which have not thus far been mentioned, have 
the simple structure of the marsupial gills, as described in the case of Quadrula, 
etc., but there are other differentiating structural features on the edges of the 
gills, which must be compared with the more primitive forms. 
2. Differentiating Structures on the edge of the gills. 
There is a difference in the structure of the edge of the inner and the outer gill, 
which is found in all our Najades, including Margaritana, and of this I shall first 
speak. 
Making vertical cross-sections through the gills of a male (See Pl. LXX XVII, 
figs. 11, 12, 15; Pl. LX XXVIII, fig. 12), we observe that the edge of the outer 
gill is simply rounded off, while that of the inner gill possesses a peculiar longitudinal 
furrow along it, which may also be distinctly seen macroscopically. As to the 
meaning of this furrow, I can not make any suggestion, and am only able to state 
the fact of its presence. By the presence of this furrow an inner may always be 
distinguished from an outer gill. 
The same difference is found in the inner and outer gill of the female (See 
Pl. LXXXVII, fig. 13), and the conformation of the edge of the gill is practically 
