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 llyria. 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 w^ere distinct genera when the number of si)ecies was small, they 

 are still distinct, and the generic basis established by Desliayes in malacology 

 and Temminck in ornithology untenable. (Desh. in Lam. Animaux sans 

 Vertebres, 6,533.) 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 Unionidse 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. Thf 

 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. Iridininm. 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 Area, and which are represented by the transverse 

 elevations upon the teeth of Castalia. Iridina is generally elongated, and 

 differs from the Unionina? in having the mantle closed posteriorly, the 

 siphons tubular, short, unequal, and without the retractor muscles of the 

 marine genera with long sij:)hons. Iridina hlainvilliaiui^ Lea {Anodonta or 

 Columha^ 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 Iridina 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 SpatJia^ Lea, 1837, and CalUscapha, Swainson, 1840. 



The genus Pleiodon was founded by Conrad, in 1834 (J. Acad. Nat. Sei., 

 vol. vii. p. 178, pi. 13, P. ovata, Swainson), for a shell from Liberia in which 

 the teeth " are alternately inserted, a generic character widely differing from 

 Iridina, which is simply crenulated or tuberculated on the margin of the 

 hinge." The teeth are further compared with those of Area 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. Etheriinm. The genus Etheria presents an irregular and very 

 variable shell, without teeth, and somewhat resembling that of Ostrea, and 



