82 GEORGE LEFEVRE AND WINTERTON C. CURTIS 
Tetragence. Marsupium occupying all four gills. 
Homogenoe. Marsupium occupying entire outer gills. 
Heterogence. Marsupium occupying only posterior end of 
outer gills. 
Mesogenoe. Marsupium occupying a specialized portion in the 
middle region of outer gills. 
Ptychogence. Marsupium occupying entire outer gills which are 
thrown into a series of peculiar folds. 
EschatigencB. Marsupium occupying the lower border only of 
outer gills. 
Diagence. Marsupium occupying entire outer gills, but dif- 
fering from, that of the Homogenae in that the egg-masses lie 
transversely^ in the gills. 
Simpson has established another group, the Diagense, for the 
genus Tritogonia but since its marsupium is constituted by all four 
gills it should at least be included in the Tetragense, if not in the 
genus Quadrula, as Ortmann suggests ('09). 
It will be seen from the above classification that three general 
conditions exist, namely one in which the marsupial adaptation, 
involves all four gills; one in which the entire outer gills only are 
utilized; and, lastly, one in which some differentiated portion of 
the outer gills constitutes the marsupial region. 
For a complete list of the genera occurring in each of Simpson's 
groups, reference may be had to his Synopsis (I. c, pp. 514-515), 
but since some of the types have not been adequately described 
and figured, we have inserted here an account of the several mar- 
supial modifications which have come under our own observa- 
tion, together with a list of the species in which we have found 
them. 
Tetragence. The marsupium in these forms comprises all four 
gills which when gravid are distended and present a smooth pad- 
like appearance. It is the condition found in the genera Quadrula 
and Tritogonia. We have encountered it in Tritogonia tuber- 
culata Branes, and in the following nine species of Quadrula: 
ebena Lea, heros Say, lachrymosa Lea, metanevra Rafinesque, 
obliqua Lamarck, phcata Say, pustulosa Lea, trigona Lea, and 
undulata Barnes. Fig. 4, which is drawn from a gravid female of 
