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 Unionidse 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 Europe, " and he was satisfied that there were ninety- 

 eight synonyms made by European authors for the single species Anodonta cyynea, 

 Drap. (Mytilus cyyneus, Linn.), and that the synonomy is nearly as profusely erroneous in 

 Unio pictorum, JJnio tumidus, Unio Batavus, and XJnio littoralis." The number of recent 

 species he has allotted to Europe is seven Uniones, one Maryaritacea, 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 x\raerica over that in 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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