116 CLASS 1. TROPIOPODA. ORDER I. BIMUSCULOSA. 



UNIO, Bruguiere. 



Testa transversa, plerumque crassa et solida, sequivalvis, ina?quilateralis, 

 epiderniide olivacea induta, interne iridescenti-margaritacea. Cardo 

 dentibus lateralibus crassis duobus, raro pluribus, in utraque valva ; 

 altero irregulari, quasi cardinali, brevi, simplici, interdum bipartite), 

 substriato ; altero antice producto, elongato. Impressio muscularis 

 postica semper composita. Ligamentum externum. 



By far the greater number of the Naiades, distinguished by their thick 

 and massive teeth, are referred to the genus Unio ; they were originally 

 separated from the Linnaean Myrc by Bruguiere ; few species, however, 

 were at that time known. 



The shell of the Unio ambiguus, in which the teeth assume a lamelli- 

 form appearance and are more than usually striated, was selected by 

 Lamarck as a type for the formation of a new genus, Castalia ; he 

 moreover associated it with Trigonia in a separate and distinct family, 

 " Les Trigonies," as establishing a link between the two proximate fami- 

 lies of the Arcacea and the Naiades. This arrangement is now aban- 

 doned, because a marked difference is evident in the animals of Trigonia 

 and Castalia ; one too is marine, the other fluviatile : and by the later 

 discovery of intermediate varieties, the latter has become inseparable 

 from the Uniones*. It is, however, but due to Lamarck to notice, that 

 in his time the shell of Castalia was one of extreme rarity, and became a 

 source of evident embarrassment to him ; for in seeking to establish an 

 affinity with the proximate genera Trigonia and Unio, he is forced to the 

 conclusion, that " cette coquille, neanmoins, ne saurait t'tre associee ni a Vun 

 ni a V autre." 



* The genus Castalia is still retained by Gray, who places it, together with the genus 

 Hyrta, in his family of IridinidcE ; we are, however, strongly inclined to question the propriety 

 of this arrangement. 



