290 ANNALS OF THE CARNEGIE MUSEUM. 
According to Clessin, differences are said to be present in the gills. 
Clessin (1876, p. 446) asserts that the tissue of the gills in Pseudano- 
donta is more delicate, that the transverse striz (‘‘Querstreifen’’) are 
more deeply incised, and that the less conspicuous longitudinal striz 
(‘‘Leengsstreifen’’) are straighter and that the breeding compartments 
(‘‘Brutfecher’’) are more quadrate (‘‘bilden vollkommenere Quad- 
rate’’). 
The tissue of the gills is indeed more delicate, which is due chiefly 
to the slighter development of the interlaminar tissue. The gill- 
filaments are finer, and the interfilamentar grooves (probably the 
“‘Querstreifen”’ of Clessin) are deeper. What he calls ‘“‘ Lengsstreifen,’’ 
is probably produced by the longitudinal rows of water pores (ostia). 
Such a striation is indeed less distinct in a face view of the gills, in 
consequence of the stronger development of the filaments. They 
are not so distinctly visible as in A. cygnea, but nevertheless they are 
present. I cannot, however, under any conditions, see that they are 
straighter than in A. cygnea; on the contrary, they are slightly more 
irregular. What Clessin means by the statement that the ‘ Brut- 
feecher’’ are more quadrate, I cannot imagine. If he means the com- 
partments formed by the septa, I can only say that, in a face view, 
they are not quadrate at all, either in Anodonta or in Pseudanodonta; 
if he means in a cross-section, they are quadrate or approximately 
quadrate only in the outer gill of the sterile female, both in Pseudano- 
donta and Anodonta, while in all other gills, non-marsupial gills of 
the male and female, and marsupial gill of the gravid female, they 
are in both genera anything else but quadrate.*8 
As has been said above, in Pseudanodonta the gill-structure is 
essentially the same as in the typical Anodonta, and all anatomical 
elements are present in both forms. The gills of A. complanata are 
indeed more delicate, chiefly in consequence of the slighter development 
of the interlaminar tissue; the gill filaments are finer, closer together, 
with deeper interfilamentary grooves, so that this layer on the outside 
of the gill is slightly thicker (compare Plate XVIII, figs. 7, 8, and 9). 
The consequence is that the rows of ostia, which are distinctly visible 
in A. cygnea in a face view, are rather indistinct in A. complanata. 
But when held up against the light, they also become distinct in the 
28 Tt almost appears as if Clessin had compared the marsupial gill of a sterile 
female of Pseudanodonta, with a non-marsupial gill‘of Anodonta. But I am not 
sure of this, since his terms are rather vague. 
