Zoological Illustrations/VolIII-Pl160



purpurascens,

Purple Anodon, or Horse Mussel.

—See Pl. 96.


 * A. testâ transversim oblongâ, crassâ, depressâ, intus purpurascente, laminâ cardinali crassâ, truncatâ, dente lamellari in utrâque valvâ supposito; umbonibus retusis.


 * Shell transversely oblong, thick, depressed, within purple; hinge plate thick, truncate, with an obsolete lamellar tooth in each valve; umbones retuse.

This is an entirely new and very rare shell, remarkable for its shape and internal colour; it is also highly interesting, as exhibiting the generic characters of Anodon, blended (in some degree) with those of Unio: according to the principles of Lamarck, it might therefore be made into a genus; but I feel convinced too much importance has already been attached by that naturalist and his followers to the hinge of bivalve shells; and that the nomenclature of the science is burthened with genera, trivial in themselves, bewildering to the scientific, and unintelligible to the student.

From having paid some attention to the Fluviatile Bivalves, and possessing a most extensive collection of specimens, I am clearly of opinion that no permanent characters will be found sufficient to retain either the genera Dipsas (Leach), Hyria (Lamarck), or Alasmodonta of Say, much less that of Damaris (Leach), and another, whose name I forget, made by Dr. Turton from the same shell as Leach's Damaris, viz. Mya Margaritifera of Linnæus. In fact, the line of demarcation between Unio and Anodon appears to rest on the first possessing cardinal teeth, and the latter having none.

I have several valves (in different stages of growth), and one perfect specimen of this shell; they were sent to me from the back settlements of North America.