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AOL XY, PROCEEDINGS OF THE NATIONAL MUSEUM. 407 
valley are found in Texas and Mexico lead me to consider this merely 
a subregion. 
Second. From the mouth of the Mississippi, extending to the southern 
extremity of Florida, northward into the British possessions, and again 
westward to include all the St. Lawrence drainage, and embracing all 
the streams in this territory that fall into the Atlantic and the Gulf of 
Mexico, there is found a set of Unionide possessing very different 
characters from those of the area first mentioned. Here the species, 
as a-rule, are moderate in size and neutral in color, and but few odd or 
striking forms are found; though Unio spinosus and U. collinus, the 
only shells known bearing true spines, belong here. The change in 
forms from Nova Scotia to Florida is very slight; in fact some groups, 
such as that of Unio complanatus, extend throughout the whole region, 
and if they can be said to break into good species, the variation is cer- 
tainly very gradual and specific lines are very difficult to determine. 
Quite a number of forms seem to be confined to the State of Florida, 
though, perhaps, when we have a thorough knowledge of their distri- 
bution, we shall find their range much extended. It is probable that 
the State was peopled with Unionide from the region just north and 
west of it; that here has been a migration as the land arose from the 
Ppdiachian chain southward and eastward. The fact that the Flor- 
ida species differ from those of southern Georgia, Alabama, and the 
lowlands of the Carolinas is due, perhaps, to a milder climate, and to 
some extent, no doubt, to the existence of a remarkable system of small 
clear lakes in the former State, in which the Unionidae have attained a 
high state of development. 
In the vicinity of the mouth of the Mississippi and for some distance 
along the territory just north of the Gulf there is a mingling of forms 
of the two areas, and the same thing is again seen in the region of the 
Great Lakes, while in the middle ground the Appalachian chain has 
acted as a much more effective barrier between the species. 
There is but a very limited development of the Unionide throughout 
the Rocky Mountain region and the Pacific slope; a couple of species 
of unios, Margaritana margaritifera, aud three or four anodontas. Sev- 
eral so-called species of west coast anodons of the group of A. Cali- 
i censie are considered by Dr. Stearns (who perhaps has a better 
knowledge of the mollusk fauna of western America than any other 
man living) mere local races of A. cygned, of Europe, a form which he 
believes to be circumboreal. (See paper on History and Distribution of 
the Fresh Water Mussels, and the identity of certain alleged species, 
Proe. Cal. Acad. Nat. Sei., Nov. 20,1882.) In this opinion I am in- 
clined to coneur. Ano. angulata, so different normally, is shown by 
photographs from specimens made by that indefatigable collector, Mr. 
Henry Hemphill, as well as by material in the National Musemn, to be 
very closely related to the species I have mentioned. The two unios 
are not stronglyharacterized, but show evident relationship to the 
species of the Atlantic drainage. I believe the Unionide of this region 
