290 MEMOIRS OF THE CARNEGIE MUSEUM 
B. Srructruray Features or tan Finer ANATOMY oF THE GILLS. 
1. The Water-tubes and the Septa dividing them. 
The following description of the water-tubes necessarily leaves out of con- 
sideration the peculiar form, Margaritana margaritifera, for this has no water-tubes. 
We have seen above that the water-tubes are formed by the interlamellar 
junctions, which run from near the base of the gill to its edge, and are generally 
developed in the shape of continuous septa, which do not differ in their tissue 
from that of the interlamellar outgrowth, that is to say, in their normal develop- 
ment their tissue is lacunar, often containing blood-vessels (See Pl. LXXXVI, 
figs. 1, 5, 6a, 7a, 10a, 12a, 13, 14a, and PI. TD XEXOGV AN tie. eS) 
This simplest structure is always found in both gills of the male, and there is 
no further differentiation of it in this sex, with the exception that toward the 
base of the gills the septa become sometimes a little more independent, being 
wider on account of the greater distance of the two lamingw from each other. The 
water-tubes formed by these septa are rather wide in the longitudinal direction 
of the gill, since the septa are distant from each other. The distances are variable, 
and often, chiefly so in the genera Alasmidonta, Strophitus, Symphynota, Anodon- 
toides, and Anodonta, they are rather large near the base of the gill, where also the 
septa become irregular, running for a short distance diagonally, and being some- 
times somewhat reticulate. In the genera just named there are often as many as 
forty and even more filaments between two septa. The average is from about 
fifteen to thirty; in other genera, the average is from about ten to twenty or 
twenty-five, although there is occasionally a larger or smaller number present. 
We may say, that normally there are more than fifteen filaments between two 
septa. 
The identical structure is found in those gills of the female, which do not 
serve as marsupia, that is to say, in the inner gill, when the outer gill alone is 
used as a marsupium, and in the inner and the anterior part of the outer gill, 
when only the posterior part of the outer gill is so used (See Pl. LXXXVI, figs. 
6a, 7a, 10a, 11a, 12a, 14a; Pl. LXXXVII, fig. 5). In those forms (genus Quadrula), 
where both gills serve as marsupia, this structure is not found at all in the female. 
(See Pl. LXXXVI, figs. 2, 4.) 
In the marsupial gills of the female important structural modifications take 
place, and these modifications, which we shall describe presently, are quite marked, 
and invariably characterize the female sex. Consequently it is possible not only 
to distinguish a sterile female (with the marsupium not charged) from a male, but 
also to discover the general character of the marsupium in a sterile female. 
