BIVALVIA. 131 
by the adductors large and deep; pedal muscle-marks distinct—one double, situate near 
- and below the oral, the other single, and above the anal adductor; mantle-mark without 
a sinus; shell nacreous; connexus ligamental. 
Animal with the margins of the mantle disunited, except between the siphonal 
openings ; tubes short—one fringed, the other plain ; foot compressed. 
Variations in the fresh-water mussels are numerous and excessive, and many 
genera, with very ill-defined lines of distinction, have been proposed for their reception. 
Unios are of a peculiar construction, being formed almost entirely of nacre; and some of 
the recent species not only display a beautiful pearly lustre, but are of a purple, pink, or 
salmon colour in the interior. Colour, Mr. Lea says, is not always to be depended upon 
for a specific distinction. 
This genus is found in the Wealden formation, and is said to have been in existence 
during the Carboniferous period. 
Mr. Lea, who is our best authority on this subject, says of this genus that there are 
already described, as inhabiting the rivers, lakes, and pools of the United States and Terri- 
tories, 465 species, to which several more in his own cabinet will have to be added; and 
he considers that there are upwards of 600 species belonging to the family Unionidee that 
are peculiar to North America. This large number is, perhaps, the more extraordinary 
when compared with the number of existing species on our own continent. Mr. Lea 
further says (‘ Proc. Acad. Nat. Sc. Philad.,’ p. 3, 1860), that he had taken great pains to 
procure specimens from all parts of Hurope, “and he was satisfied that there were ninety- 
eight synonyms made by European authors for the single species Anodonta cygnea, 
Drap. (Mytilus cygneus, Linn.), and that the synonomy is nearly as profusely erroneous in 
Unio pictorum, Unio tumidus, Unio Batavus, and Unio littoralis.” The number of recent 
species he has allotted to Europe is seven Uniones, one Margaritacea, one Monocondylea, 
and one Anodonta. This great discrepancy between the numbers on the two continents gives 
to North America a preponderance of sixty-fold over those of Europe. The extent of fresh 
water in the rivers and lakes of America may in some degree be assigned as a cause for 
the very great development of species in that continent over those in Europe; but that 
alone appears insufficient for explanation, as the proportions of fresh water between the 
two continents will not bear a comparison with this disproportion of species, more espe- 
cially when we consider that it is principally on the shores of these extensive lakes, or at 
least in not very deep water, that we are likely to find living many of the species, and 
can hardly take into our computations the fresh-water area of North America. This 
would materially reduce the apparent excess in the area of feeding-ground for these animals 
in America over that m Europe, and it does appear to me that some other cause than 
mere difference in the superficial extent of the medium in which they live, is required to 
account for this great difference in the relative number of the species found in the two 
Continents. We have, according to Mr Lea, ninety-eight synonyms in a single European 
species, thereby implying very great variation; but to what extent we are permitted to 
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