THE NAUTILUS. 19 
and others have placed in this genus, but I believe they have no 
close relationship to these South American forms, and that they are 
merely depauperate Unios, which have a close affinity to species of 
that genus found with them. 
The Unionide of Europe, North and Central America, and prob- 
ably of the whole Northern Hemisphere, develop eggs in the outer 
gills alone as far as is known, with the exception of Unio multiplica- 
tus and one or two others, which contain embryos in all four leaves 
of the branchiae. Ihering states that in all the Najidae hitherto 
examined from South America, the eggs are borne in the inner 
gills. I may remark in passing that the shells of the Australian, 
New Zealand, and many South African Unios bear an astonish- 
ing resemblance to those of South America in form, texture, smooth 
epidermis and concentric, sometimes slightly granulated sculpture, 
and especially in the peculiarly compressed, parallel cardinal teeth, 
and Suter states’ that the embryos of N. menziezi are borne in the 
inner gills. Ihering calls attention to the fact that all South Ameri- 
ean Unionide have a radial beak sculpture, and suggests that prob- 
ably the same character may be found in the New Zealand species. 
I have carefully examined extensive series of Unio menziezi and 
lutulentus, and on the latter find that the umbos are radiately 
ribbed where the shells are not too badly eroded, and there are 
traces of such ridges on the former and on some Australian species. 
He believes that we shall find the beak sculpture one of the best 
characters for determining the minor divisions of the Unionide. 
Notwithstanding the opinion of this eminent conchologist, and the 
fact that Mr. Wm. A. Marshall, of the New York State Museum, 
who has also given this subject some very careful study, believes that 
the beak sculpture is quite constant and may be used in determining 
species, my own experience in handling great quantities of material 
from all over the world leads me to consider this a somewhat varia- 
ble character, and although it will no doubt prove very useful in 
studying species and the smaller groups, yet I am sure it cannot by 
any means be always relied on. 
It is only in Europe that the post-embryonic larve of the Union- 
ide have been observed actually attached to fishes, though the 
North American species are known to possess hooks and bristles 
during this stage, and they no doubt make use of the same means to 
assist in their distribution, as do their Old World relations. 
3. Z. Jl. of Science, No. 6, Vol. I (new issue), p. 250. 
