REPRODUCTION IN THE UNIONIDiE 
85 
piratory portion either by a deep notch or a distinct fold and pro- 
jecting further down into the mantle chamber, as it is much deeper 
dorso-ventrally than the non-modified anterior end of the gill. 
Unlike that of the Tetragenae and Homogense, the marsiipium in 
the Hertogenae is readily recognized when empty, for, in addition 
to its permanent demarcation, its walls are thinner and more mem- 
branous in appearance than those of the respiratory portion, and 
after discharge of the glochidia it is seen as a flabby collapsed 
pouch. The inflation of the posterior end of the valves which 
is characteristic of the females in the Heterogenae is associated 
with this type of marsupium, as it allows of increased space inside 
the shell for the accommodation of the gravid gill. Owing to this 
enlargement of the shell, which is absent in the males, the sexes 
are externally distinguishable in the Heterogenae. 
The typical condition is seen in the genus, Lampsilis, as shown in 
fig. 2, which is drawn from a gravid individual of L. subrostratus. 
The extent of the marsupial modification, however, varies in differ- 
ent genera and even in different species of the same genus, occupy- 
ing, as already stated from about one-third to two-thirds of the 
length of the entire outer gill. Fig. 5 illustrates the marsupium in 
L. rectus, in which it forms the posterior third of the greatly elon- 
gated outer gill and is sharply marked off from the resoiratory 
region by a deep notch. The last two figures indicate me two 
extremes of the differentiation in this group. Simpson has 
included fourteen genera in the Heterogenae, only three of which, 
however, have come under our observation, namely Lampsilis, 
Obovaria, and Plagiola. We have recorded this type of marsu- 
pium in Lampsilis (Proptera) alatus Say, anodontoides Lea, gra- 
cilis Barnes, higginsii Lea, laevissimus Lea, ligamentinus Lamarck, 
luteolus Lamarck, rectus Lamarck, subrostratus Say, and ventri- 
cosus Barnes ; in Obovaria ellipsis Lea ; and in Plagiola elegans Lea 
and securis Lea. 
MesogenoB. This group is so designated by Simpson to include 
two genera, Cyprogenia and Obliquaria, in which a variable num- 
ber of enlarged water-tubes in the middle region of the outer gill 
are specialized as the marsupium, the portion of the gill both in 
front and behind retaining its ordinary respiratory character. We 
