UNIO Pl. 1.
U. truncatus
In our former Series, we have frequently mentioned the Fluviatile Bivalve Shells, of which the genus Unio appears to be the type. In a group which present so few certain characters, either for discriminating the species or characterizing the genera, we have, with other writers, held different opinions at different times. And the reader has only to peruse an interesting paper on these shells, in the Zoological Journal (Vol. I. p. 53), to be convinced of the intricacy of the subject.
UNIO truncatus.
Truncated River Mussel.
Acephala Dimyaria.
Sub-family Les Nayades.—Lam.
Synopsis of the Genera.
Unio. Shells with lateral and cardinal teeth, the latter short and deeply divided.
Hyria. Lateral and cardinal teeth distinct; but the latter lengthened, and united to the former by irregular dentations.
Iridina. Teeth consisting of a single crenated line, parallel with the ligament.
Anodon. All the teeth either obsolete, or entirely wanting.
Alasmodon. Lateral teeth none; cardinal teeth simple, or slightly divided.
Specific Character.
Shell thick, oblong, sub-cylindrical, within pearly, umbones close to the anterior margin, which is truncate; posterior extremity narrowed.
Nevertheless it is observed, by those who study natural affinities, that when the links between two distinct forms of animals are so complete, that their dissimilarities are lost in intermediate and undefinable gradations, it is then that the natural arrangement is most likely to be discovered. For it has been demonstrated in such groups, that the most perfect order and harmony will come out, of what appeared an inextricable entanglement of relations. Such, however, will never result from the belief in a simple scale of Nature, or by attempting to circumscribe groups by absolute characters. We therefore now offer the foregoing sketch of the natural divisions of the Nayades, as the result of all we have seen or read upon this difficult subject.
We are unacquainted with any described species to which the shell here figured can be referred. Its substance is very thick, and its form nearly cylindrical. We have seen but one specimen, and that was with Mrs. Mawe.