288 MEMOIRS OF THE CARNEGIE MUSEUM 
if there is only a small opening left, this is never placed anteriorly, but always at 
the posterior end of the foot. 
According to these observations, I must attribute to this character a certain 
limited systematic value. There are a number of forms, in which the primitive 
condition of an unconnected, free inner lamina of 
the inner gill is always found. There are other 
forms, which belong to the more advanced types, 
in which this lamina is always entirely, or almost 
entirely, connected with the abdominal sac. Be- 
tween these stand others, in which all transitional 
conditions are found, and in some of these an in- 
termediate condition seems to be characteristic, 
while in others this character is variable. 
As has been mentioned above, posteriorly to 
the abdominal sac and foot, the inner laminz of 
the two inner gills unite in the median line of the 
body (See fig. 8). By this union, together with 
the connection of the outer laminz of the outer 

gills with the mantle, a complete separation of the 
branchial chamber from the posterior part of the 
Fic. 3. Anodonta grandis gigantea (Lea) 
Left half of body, seen from below, with gills suprabranchial canals (cloacal chamber) is effected 
removed; showing attachment of gills.—plp, 
Sea tc eee Une ene eee by a septum or diaphragm, which forms a hori- 
attachment of outer lamina of outer gill to zontal division, from which the gills hang down 
me omer meni (ee fig. 4). At the posterior end the four supra 
latter; be, line of connection of the two inner branchial canals of the two gills of the right and 
laminw of right and left gill; 0, sexual and eft, side are united into one’ cavity, stlte scloacal 
chamber. This is brought about by the fact that 
the gills extend posteriorly beyond their attachment to the body at their base, or 
rather, that this attachment (ctenidium) does not extend to the posterior end of 
the body (See fig. 5). This attachment, the common base of the two primary 
limbs of the gills on each side, runs backward to a point, which generally lies 
about midway between the posterior end of the foot and the posterior margin of 
the mantle; beyond this point the inner limbs of the two gills on each side remain 
connected with each other, but are no longer attached to the body, and the 
two suprabranchial canals on each side are fused (See figs. 3 and 5). Thus of the 
four suprabranchial canals present in the anterior part of the body, each of which 
belongs to one gill, the two median canals are united first immediately behind the 
renal orifices. 
