THE NAUTILUS. 93 
heavy in crassidens, and the teeth very strong and of the same type 
with the other species of group B, while the decided downward 
curvature at the posterior end in old specimens approaches both 
species to the same, and removes them decidedly from A. 
In U. multiplicatus the female was found November 1, with evi- 
dently mature embryones filling the uterus sacs, which are of rather 
the same appearance as those of U. alatus, and occupying the most 
posterior part of the outer branchiz. Also, otherwise this species is. 
quite different from U. undulatus Barn., in spite of the similar ap- 
pearance of the old mussels, as already pointed out by Say. 
A few words about U. alatus Say, laevissimus Lea and gracilis 
Barn. may be added. Of all three the branchial uteri were found 
filled late in October, in the former evidently discharging the em- 
bryones, and of a rather different formation from that in the two 
others, while the young mussels of the two former species are very 
much alike. It is known that in U. alatus the female mussel is 
markedly produced downward, and more so in gracilis, while in 
laevissimus it is searcely distinct. from the male. 
Besides these two main, and, as it seems, most numerous groups, 
there are some species of quite different types. In order not to let 
this article become too lengthy, they, together with Margaritana and 
Anodonta will be considered later. 
It is probably known to most conchologists that during the first 
two or three years of life, the genital glands are not developed at 
all. Yet the specimens are still rather small and young when they 
begin producing ova and sperma, and the assertion that the presence 
of embryones is a criterion of maturity of the parent is as far from 
truth here as it is for the Cycladide, In group A the filled uterus- 
or embryo-sacs increase in numbers as well as in size with advancing 
age of the mother animal. In U. subovatus Lea, e. g., 18, 25,28, 40 
have been counted on a side, in specimens of different sizes, and 
these are probably not the lowest and highest numbers to be found, 
and correspondingly so in other species. For every one of them 
there is, however, an average number the rule for an adult, and 
these numbers are very different for the different species. 
The embryones, or glochidia, of many species have been exam- 
ined and figured long ago, especially by Lea, and they prove to be 
of rather different forms. Very probably they would be found more 
or less uniform in the several groups, and with the latter show 
differences which may also be of value for systematics. Although 
