ORTMANN: A MONOGRAPH OF THE NAJADES OF PENNSYLVANIA 287 
constant. Thus in the species placed by Simpson in the genera Quadrula, Pleuro- 
bema, Unio, Margaritana, Symphynota, Anodontoides, and Anodonta, I always found 
the inner lamina of the inner gill entirely free from the abdominal sac, with the 
exception of its anterior end. This is the more remarkable, since both Lea and 
Simpson (See Simpson, 1900, p. 766, footnote 2) affirm that in Quadrula multiplicata, 
a species which stands near our Q. undulata, the inner gills are sometimes free, 
sometimes connected. I have not seen that particular species; but of Q. undulata 
I have examined some thirty specimens, and have never seen a connected inner 
gill. Although the number of specimens investigated in this case seems small, 
the fact that I never found a connected inner gill in the genera named above, is, I 
think, rather significant, for I have looked over about four hundred individuals 
belonging to these genera, and it would be very strange, if variations occur here 
at all, that I by a curious chance never came across a single one. 
Further, I found the inner lamina of the inner gill always almost entirely 
free in the following species of other genera: Alasmidonta heterodon (fifteen speci- 
mens investigated) Lampsilis parva (three specimens). In Obliquaria reflexa (four 
specimens) the gill is also free, but for a shorter distance, a little more than half 
of the abdominal sac. The same is the case in Plagiola securis (six specimens), 
where the length of the free edge is somewhat variable (in one case the slit was 
very short, but only on one side!). 
On the other hand there are forms, in which the inner limb of the inner gill 
is always entirely united to the abdominal sac. This is the case in the genus 
Alasmidonta (excluding A. heterodon), in Obovaria ellipsis, circulus, and retusa, in 
the genus Truncilla, and in many species of the genus Lampsilis (gracilis, alata, 
recta, orbiculata, ligamentina, radiata, luteola). 
In all those not hitherto mentioned, there is variation in this respect. This 
is most evident in the genera Strophitus and Ptychobranchus. In fact in the latter 
genus I have observed all conditions, from an inner lamina almost entirely free 
to an inner lamina entirely connected. In Strophitus it varies from free for about 
half the length of the abdominal sac to entirely connected. In Plagiola elegans 
(two specimens) the open slit was in one case short, about one fourth of the length 
of the abdominal sac, and in the other there was only a small hole at the posterior 
end of the foot. In Lampsilis iris, nasuta, multiradiata, cariosa, ventricosa, and 
ovata, the inner gill is entirely connected, or has a small hole at the posterior ex- 
tremity of the foot; and in Micromya fabalis (four specimens), the slit is short, 
from about one fourth to one half of the length of the abdomen. 
The closing of the slit proceeds always in an antero-posterior direction, and 
