84 GEOKGE LEFEVRE AND WINTERTON C. CURTIS 
ing their embryos or glochidia which were often expelled with con- 
siderable force through the exhalent siphon. This behavior was 
so characteristic of the genus that, in order to make a correct deter- 
mination of the condition of the marsupium, it was necessary to 
open Quadrulas immediately after taking them from the water. 
When this was done, all four gills were invariably found to be 
charged on opening females which contained embryos in pre- 
glochidial stages, that is, at any tim.e before normal spawning had 
occurred. The habit of aborting embryos, when disturbed has, 
been observed by us in but one species not belonging to Quadrula, 
namely, in Unio complanatus, which has been repeatedly seen in 
th act of discharging the contents of the marsupium shortly after 
being placed in the aquaria of the laboratory at Woods Hole, Mass. 
In all likelihood it occurs in other species of Unio, and it has been 
referred to by Schierholz ('88) and Latter ('91) as taking place in 
Anodonta. It is probably due, as Schierholz suggests, to imper- 
fect aeration of the water, and if this is true, one would not expect 
to find it in those mussels which have a differentiated marsupium, 
like the Heterogense, since in such forms the respiratory and mar- 
supial functions of the gills are not so intimately associated. 
HomogencB. The condition in which only the outer gills are 
utilized as a marsupium is present in sixteen genera according to 
Simpson. We have verified its occurrence in Alasmidonta truncata 
Wright; Anodonta cataracta Say, grandis Say, implicata Say; 
Arcidens confragosus Say; Pleurobema sesopus Green; Symphy- 
nota complanata Barnes, costata Rafinesque; and in Unio com- 
planatus Dillwyn and gibbosus Barnes. The outer gills when filled 
with embryos or glochidia may be greatly distended beyond their 
normal dimensions, and in this condition are enormously swollen 
pad-like structures filling a large portion of the mantle chamber. 
Fig. 6 represents the marsupium of Symphynota complanata 
which may be taken as typical of the class. 
Heterogence. In this group the marsupium occupies only the 
posterior portion of each outer gill, varjdng in extent from about 
one-third to two-thirds of the entire length of the latter. It is per- 
manently differentiated, being sharply marked off from the res- 
