AMPHIDESMA. 



Lam. — Hist. Nat. des Anim. sans vert. v. p. 489. 



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TESTA subinaequilatera^ transversa, subovalisvel 

 rotundata: ( inter dum laterihus subhians : ) Cardo 

 utriusque valvae dente unicOj vel dentibus duobus ; 

 dentibus lateralibus valvae alterius^ contiguis^ 

 distinctis ; alterius obsoletis. Ligamentum dii- 

 pleX;, externum tenue^ breve; internum foveolis 

 cardinalibus elongatis^ affixum. hnpressiones 

 musculares duae^ distantes^ suborbiculares. Sinus 

 impressionis pallii adhaerentis musculi magnus. 



Established by Lamarck, as he informs us, at first under 

 the name of Donacilla, because the species with which he 

 was first acquainted, had the aspect of a Donax. The 

 name, Amphidesma, which he has since i^^iven to it, is taken 

 from its great peculiarity, which consists in its having one 

 portion of the Ligament external, and the other, generally 

 far the greater, entirely within the shell ; for the ligament 

 in this genus, as well as in all regular bivalves, consists of 

 two distinct portions ; the great difference between this 

 and other bivalves being in the position of the inner portion 

 at a distance from the outer ; and in this respect it ap- 

 proaches near to the Mactracees^ and the genus Lutraria 

 particularly, from which indeed it is only distinguished by 

 its lateral teeth, and by the valves being close all around, 

 whereas in Lutraria they gape at one end. These appear 

 to us to be the principal distinctions between Amphidesma 

 and Lutraria^ we are consequently of opinion that Amphi- 

 desma should be arranged near to or with the Mactracees^ 

 and indeed we find several of Lamarck's species arranged 

 by Linneans with the Mactrae. We know not what con- 

 siderations have induced Montagu to add several of them 

 to his genus Ligula, of which they do not possess the 



