THE NAUTILUS. 19 
very interesting statements regarding their anatomy. He places 
those species in which the young are carried in the posterior part of 
the outer branchix, and the female shell is inflated in the post-basal 
region, in a group designated as A, which he states, as a rule, have 
bright shells, and are gravid from late summer to winter. In group 
B all four branchix are charged throughout their length ; the shells 
are generally dull colored, and do not show marked differences be- 
tween male and female. 
In my earlier attempts at classifying the Naiades, I based my ar- 
rangement almost wholly on shell characters. I did this because in 
the splendid collections of the National Museum we had either the 
types or authentic specimens of a very large proportion of the 
known, valid species, and because I was anxious to see whether a 
classification could be based on the shells alone that would be sup- 
ported by the evidence of the anatomy. I have since then ex- 
amined the soft parts of a large number of species, and carefully 
tabulated the results of the work of Lea and others, and I am ex- 
ceedingly gratified in being able to say that in almost every case the 
characters of the animal and shell seem to essentially agree. Ina 
few instances, the evidence of the former has thrown important light 
on relationships which could not be determined with certainty from 
shell characters alone, and vice versa. In some cases, where there at 
first seemed to be a contradiction, more careful study has shown es- 
sential agreement. 
There are certainly two great groups of North American Unios. 
In the first the shell is generally, though not always, covered with a 
hard, smooth, bsight epidermis, which is often rayed or marked 
with patterns of attractive color. It is rarely sculptured with any- 
thing beyond slight concentric ridges, and in only a few instances 
has it any ridge on the posterior slope, and, with possibly one or two 
exceptions, the outline is never arcuate, even in old age. The fe- 
male shell is usually decidedly swollen in the posterior basal region 
to accommodate that part of the outer branchiz which contains the 
embryos. The beak sculpture is generally delicate, consisting, for 
the most part, of close, fine, paralled ridges, which have a tendency 
to fall into an anterior and posterior loop, the latter sharp pointed 
below. 
The embryos are contained in the posterior part of the outer 
branchiz. The ovisacks are distinct, being sepurated by a suture, 
and the whole marsupium is rounded below, projecting generally, 
