THE NAUTILUS. 29 
an isolated cavity, which is evidently corresponding with, and de- 
scendant from an ovum. These cylinders seem to be not homologous 
with the “sacks” in Lampsilis and other groups, and may properly 
be called placente. When removed from the branchiz and sur- 
rounded by water, they swell up, at the same time becoming more 
translucent, aud each embryo is dislodged from its cavity, evidently 
expelled by the swelling of the surrounding substance, and the exit 
facilitated by its softening. But each one is still hanging on the 
cylinder, held by a short byssus thread, whose proximal end is 
attached to the soft parts of the young, the distal to the inner lining 
of the ovum cavity. Very probably these placentz are discharged 
as such by the parent, with the young first enclosed, and then 
attached for some time. 
In the other species named above, the arrangement is rather 
similar; the young are attached to and held together by filaments 
which seem to be homologous with the placentz of edentula. And 
the same byssus has been seen in the young of marginata, coiled up 
at the distal end. 
The embryonic young of these species as well known, are all of 
the same type, 7. e. pointed below and strongly “hooked,” quite 
different from those of other groups, a very significant character. 
The shells of the adult show some common features, and their nacre 
is of rather the same appearance. All these qualities combined seem 
to prove that the several species under consideration, with some 
nearly related forms, constitute a rather well characterized genus. 
That the hinge of edentula is still more rudimentary than that of 
most of the others, can be no valid argument to the contrary, and 
also the more developed placentz are, in my opinion, of secondary 
significance. 
3. Gonad and gravid branchice in the young and old; Parasite.— 
It has already been stated that in young individuals, two, three, or 
possibly four years old, the gonads are not yet developed at all, and 
at that period the shells show no distinction of sexes, even in Lamp- 
silis. The animals seem to be asexual and, in this respect may be 
regarded as larve. There are very few animals, of higher or lower 
order, showing this peculiarity in their apparently definite state, 
except possibly some of their congeners, marine Pelecypods. When 
the gonads commence growing, there are at first few acini developed, 
producing small quantities of cither ova or sperma. It is a ques- 
tion, however, whether at that juvenile age the future sex of an in- 
