408 NOTES ON FLORIDA UNIONIDA[—SIMPSON, : 
to be much more intimately connected with those of the Atlantic sys- 
tem than the Mississippi valley, and L consider this a subregion of the 
former. Perhaps in the whole United States not over one hundred 
valid species will be found to be extra limital. It is worthy of note 
that while these areas seem to confine the Unionide within given 
bounds, they present no perceptible barriers to the Corbiculide@ or the 
fresh-water wnivalves which are everywhere found with them. 
Lea’s arrangement of the Unionide in two great divisions—symphy- 
note and non-symphynote; the former including those species in which 
the dorsal margin is more or less produced into a wing, the latter those 
without it—and the subdivision of these groups into smaller ones, first 
on peculiarities of sculpture, and secondly according to form, is a 
simple, comprehensive one, but it is an arrangement which is certainly 
artificial and not in accordance with all the facts of nature. We often 
find shells which are entirely destitute of the dorsal wing, which by 
sculpture, form, texture, and general characters are evidently closely 
related to those which are symphynote, and there is even the greatest 
possible variation in this character in the individuals of the same species. 
Lea himself acknowledged that this character was of little value in 
Classification. The same species may be either absolutely smooth or 
strongly pustulate, as in Unio infucatus, or U. pustulosus, which, in 
the variety Schoolcrafti, is often entirely destitute of nodwes; if may 
be strongly sulcate or entirely plain, as UU’. negatus, a shell with well- 
marked concentric ridges, wlich graduates imperceptably into U. ru- 
biginosus, a pertectly plain form; and U. Lstabrookianus, stramineus: 
and others exhibit precisely the same variations. So far as form goes 
as a character by which to classify, Mr. Lea has, in order to conform to 
it, been obliged to widely separate members of such well-characterized 
groups as those of U. Buckleyi, U. complanatus, and U. luteolus. 
In arranging I have attempted to place the species in natural groups, 
putting those together as far as possible which seem to have a more 
intimate relationship, and which have probably sprung from a common 
ancestry at a recent date, and yet doing as little violence as possible to 
Mr. Lea’s general arrangement. Lam aware that these groups are not 
as a rule of great importance, that they often imperceptibly merge into 
others, and that any lineal arrangement of them must be artificial, as 
many of them show about equal relationship to several others. In 
grouping, sculpture, form, color of epidermis and nacre, the teeth, gen- 
eral facies, and texture, as well as locality, have all been taken into 
consideration, 
I have for the most part given a set of outlines drawn from the valves 
of the shells, to illustrate the form and variation and to assist in the 
determination of the species. In a paper like this the cost of figuring 
so large a number of forms would be too great, and it is believed that 
these outlines, often drawn from the type-shells, which I have found 
very useful when sent to students, will prove a great aid in comparing 
with specimens, giving an accurate idea of dimensions and form, I 
