304 ANNALS OF THE CARNEGIE MUSEUM. 
the edge is here sharp. The water-tubes (ovisacs) have a peculiar 
curve backward toward a point near the posterior end of the mar- 
supium. For the present, I cannot correlate this structure with any 
special function, and must leave it for future study to decide what the 
meaning of this feature is. 
We may arrange the genera of this subfamily as follows: 
ai. Marsupium thin, not kidney-shaped, ovisacs subcylindrical or very slightly 
compressed. Placentz generally rather solid. Inner edge of mantle in 
front of branchial opening not distinctly differentiated. Shell rounded, 
ovate, or subelliptical, sometimes with outer sculpture. Male and female 
shells practically alike. 
hi. Marsupium occupying the whole of the edge of the outer gill, folded. Pla- 
cente subcylindrical, club-shaped, short, Shell subelliptical. 
Ptychobranchus. 
b2. Marsupium occupying only a part of the outer gill. Placenta moderately 
elongated or very long, subcylindrical, or very slightly compressed. 
Shell more or less rounded, generally with tubercles. 
a. Marsupium consisting of comparatively few, elongated ovisacs, ex- 
tending from near the base of the gill beyond its edge. Placente 
subsolid, subcylindrical. 
di. Placente moderately long, slightly curved. Marsupium just behind 
theymuiddlesonthe (gillian re ee Obliquaria. 
dz. Placente very long, spirally coiled up. Marsupium in, or slightly 
inmront of they middlerorf theres ere eee Cyprogenia. 
¢. Marsupium consisting of a large number of ovisacs, occupying the 
larger posterior section of the outer gill. Placentz slightly com- 
pressed near base, tapering, and becoming subcylindrical toward 
the margin. 
d;. Ovisacs straight. Placente almost entirely beyond the edge of the 
gill, subsolid. _Glochidia much longer than high, arranged on the 
outer faces of the marsupium. Edge of marsupium blunt, in old 
specimens warped and folded. Shell rounded, disk nodular. 
Dromus. 
dz. Ovisacs strongly curved back in the distal part. Placente for a 
great part enclosed between the original gill lamina, not solid. 
Glochidia not longer than high, distributed uniformly through the 
placental mass. Edge of marsupium sharp, pointed behind, not 
folded. Shell subelliptical, disk smooth............. Friersonia. 
a2, Marsupium thick, more or less kidney-shaped, ovisacs dilated and compressed. 
Placentze not very solid. Inner edge of mantle in front of branchial 
opening, more or less differentiated. Shell rounded, elliptical, or elon- 
gated, swollen or compressed, generally without any outer sculpture. 
Male and female shells more or less different in shape. 
by. Inner edge of mantle slightly lamellate and crenulated, but without dis- 
tinct papilla or flaps. Male and female shell differing only slightly 
in shape, sometimes hardly at all. 
