20 THE NAUTILUS. 
especially when filled with young, below the inner gill. A slight 
fold commonly runs around it near the base, and parallel with it, 
which is often seen even when the whole is distended with young. 
The specialized marsupium of this group may be easily detected, 
even when it is empty, and when full it isa most beautiful object, 
the bases of the ovisacks being often rounded and colored. There 
are three or four, perhaps more, groups of this great division; one 
typified by such oblong, smooth forms as Unio anodontoides, luteolus, 
cariosus and levissimus; another in which the inflated part of the 
shell is of different texture from the rest, is often distinctly marked 
out, and sculptured with radiating ridges ending in teeth at the 
edge, including Unio perplexus, sulcatus, brevidens and the like; a 
third containing short forms with a distinct posterior ridge and re- 
markably painted epidermis, such as Unio securis, donaciformis and, 
perhaps, caperatus and dromus. 
This great group is certainly entitled to generic rank, and the 
divisions I have indicated may perhaps be made into subgenera. I 
believe that the name Lampsilis, proposed by Rafinesque, and after- 
wards used by Agassiz, may be applied to this genus. 
The second great group contains forms in which there does not 
appear to be any special differentiation in shells due to sexual char- 
acters, and which are true Unios. In fact I consider the question 
as to the distinction or separation of the sexes in the true Unios 
and Anodontas far from being settled, although it is one which has 
been fought over since the time of Leuwenhock until the present. 
A number of excellent authorities have declared, after making many 
careful dissections, that the sexes of these forms were separate ; 
others equally capable have concluded that they were united, others 
that the earlier stage was that of a male and later on a female, 
while still others claim to have found the sexes united in some in- 
dividuals and separated in others. 
The shells of this great group are usually rather dull in color ex- 
ternally, they often have a decided posterior ridge, and generally 
become arcuate in outline in old age. The beak sculpture, as a 
rule, is rather coarse and irregular, in most cases consisting of a few 
nearly straight bars, which are thickened where they pass over the 
posterior slope. At the extreme anterior and posterior dorsal por- 
tions of the young shell there are often found fine, radiating ridges, 
which sometimes pass below into the heavy, horizontal undulations. 
The embryos are distributed throughout the whole length of the 
