UNIO truncatus. 
Truncated River Mussel. 
entire ———_——— 
Acephala Dimyaria. 
Sub-family Les Nayades.—Zam. 
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 length- 
ened, and united to the former by irregular dentations. 
Irmp1na. Teeth consisting of a single crenated line, parallel with 
the ligament. 
Anopon. All the teeth either obsolete, or entirely wanting. 
AtasMopon. 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. 
a EE i 
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 charac- 
terizing 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. 
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 dis- 
covered. 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. 
Unio Pl. 1. 
