MOLLUSCA. 71 
and Trigonia in form, and to the latter by some affinity in the teeth. It was 
placed next to Trigonia by Lamarck, but an examination of the mollusc 
shows that it is allied to Unio, and the same remark applies to Ilyria. The 
propriety of making a single genus of all these is doubtful, genera being thus 
made to depend upon the number of species, because the more numerous 
these are, the more links will there be between dissimilar forms. If Castalia 
and Unio were distinct genera when the number of species was small, they 
are still distinct, and the generic basis established by Deshayes in malacology 
and Temminck in ornithology untenable. (Desh. in Lam. Animaux sans 
Vertébres, 6,523.) Some might extend these views to species, and deny 
specific distinctions in those genera which have many species, and this has 
been done by Lamarck in Cerithium, but he is consistent in extending his 
views to species in general. 
Some have gone so far as to assert that the family of Unionidee contains 
not only a single genus but a single species. Without inquiring how the 
term species is to be limited, we feel confident that most of the Uniones can 
be determined specifically, as well as most genera of conchifera. The 
spines of Unio spinosus and U. collinus are often lost with age, and yet 
there is no difficulty in distinguishing them. U. trapezoides was confounded 
by Lamarck with one or two other species, yet no great knowledge is requisite 
to distinguish it. Deshayes doubts the distinctness of Unio lanceolatus, yet 
a very slight examination is sufficient to show that it is different from all 
others. Unio subtentus, viridis, cylindricus, heterodon, and a host besides 
without prominent characters, can always be distinguished. 
Sub-fam. 2. Iridinine. This sub-family contains some shells which have a 
great resemblance to Anodonta, and others which have teeth upon the hinge 
margin much as in Arca, and which are represented by the transverse 
elevations upon the teeth of Castalia. Iridina is generally elongated, and 
differs from the Unioninz in having the mantle closed posteriorly, the 
siphons tubular, short, unequal, and without the retractor muscles of the 
marine genera with long siphons. Jridina blainvilliana, Lea (Anodonta or 
Columba, Am. Phil. Tr. v. 77, fig. 35), may be an exception, because the 
unusual curve in the pallial impression indicates a considerable power to 
retract the siphons. 
The genus Jridina was established by Lamarck upon I. exotica (also 
named nilotica and elongata) in which the hinge is smooth, but acquires 
granulations with age, when it conforms to Lamarck’s characters. It is 
also named Spatha, Lea, 1837, and Calliscapha, Swainson, 1840. 
The genus Pleiodon was founded by Conrad, in 1834 (J. Acad. Nat. Sci., 
vol. vii. p. 178, pl. 13, P. ovata, Swainson), for a shell from Liberia in which 
the teeth “‘ are alternately inserted, a. generic character widely differing from 
Tridina, which is simply crenulated or tuberculated on the margin of the 
hinge.” The teeth are further compared with those of Arca and Nucula. 
In a specimen now before us there is a flat internal ligament at the anterior 
internal extremity of the teeth. 
Sub-fam. 3. Htheriine. The genus Etheria presents an irregular and very 
variable shell, without teeth, and somewhat resembling that of Ostrea, and 
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