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abdominal sack, the posterior extremity not attached to the mantle for half an inch, 

 the point standing out entirely free, but from the point of attachment a filament 

 extends to the margin. Palpi large, subfalcate, thin, united more than half way down 

 the posterior edges. Mantle thick, much thicker and colored along the edges ; on the 

 outside it is whitish or light brown, on the inside, posteriorly, it is quite blackish .* 

 Branchial opening very large, black, with very beautiful branching arborescent 

 papilla?. Anal opening very large, black and without papilla?. Super-anal opening 

 very long, blackish along the inner edges, not united below. The foot is dark colored 

 and partly blackish brown. When the mantle is thrown back, nearly the whole mass 

 within is blackish, the posterior part being quite black. Those parts of the siphons 

 and mantle protruded, when living, as represented in the plate, are entirely black or 

 brownish black. The adductor muscles are very large, as are also the the tractors, 

 and these leave deep cicatrices in the nacre, which in the old ones are very much 

 corrugated. The dorsal muscles are attached in the cavity of the beaks, posterior to 

 the cardinal teeth. The palleal cicatrices are well impressed, and the middle part of 

 the mantle being attached to the shell, there are many ventral cicatrices in the cavity. 

 The JJnio monodontus Say, properly belongs to this genus. The soft parts have the 

 same character, and the hard enveloping parts are very closely allied. 



RemarlxS. — This interesting mussel, which has been so well known to Zoologists, 

 ancient and modern, has never been, as to its soft parts, well described nor well 

 represented. In the plate I have merely represented the fringes, of which I made a 

 drawing at the moment I found them so perfectly extended beyond the margin of the 

 valves. They are very remarkable, and differ from any species of the Unionidce with 

 which I am acquainted. The figure simply represents the form of the fringes, 

 with an accurate outline of the shell and extruded- foot. The description of the soft 

 parts is made above with care and exactness. This species is remarkable for its 

 immense geographical distribution. I have it from England, France, Belgium, 

 Germany, Denmark, Norway, and I believe it is found in nearly all parts of Northern 

 Europe. On this side of the Atlantic we have it in the New England States and as 

 far south as middle Pennsylvania. Recently I am in possession of a specimen from 

 Lake St. John, Gaspe, Lower Canada, by the kindness of Sir "Win. E. Logan, Chief of 

 the Geological Survey of Canada. On the western side of the American Continent it is 

 again found, and I owe to the kindness of the officers of the Smithsonian Institution 

 and to Dr. Trask, Geologist of California, the advantage of examining several specimens 

 in alcohol, from California and Oregon, all of which presented the same characters as 

 ours, except that in the color of the nacre those from California and Oregon were 

 purple. Mr. Nuttall, more than twenty years since, gave me specimens which he had 

 brought from Oregon. It is this variety which Dr. Gould has called Alasmodonta falcata. 



*The prevailing black color is in part owing to a superficial pigment, which is partially removable. 



