U N I O nasutus. 

 Rostrated River- Mussel. 



Generic Character. 

 Testa transversa, aguivalvis, non qffixa; natibus decortkatis, suberosis ; impressio 

 muscularis postica composita. Dens cardinalis unicus, brevis, irregularis^ 

 simplex aut bipartitus, substrkitus; denies laterales duo, elongati, com- 

 pressi, infra pubcm producti. 



Typus Genericus Mya Pidorum. Linn. 

 Shell transverse, equivalve, not affixed, the tops decorticated. Posterior 

 muscular depressions double. Cardinal tooth one, short, irregular, 

 simple or double, striated ; lateral teeth two, elongated, compressed, 

 and prolonged beneath the corslet. 



Generic Type M\fa Pictorum. Linn. 



Specific Character. 



U^(Div. Q.) testa transverslm elongata, niargine dorsi recta, antict angulatd 

 oblique attcnuuta, extremitate subtruncata. 



Unio (Div. 2.). Shell transversely elongated : dorsal margin straight ; an- 

 terior side angulated, obliquely attenuated, the extremity slightly trun- 

 cated. 



Lister, tab. 151. Jig. 6. 



Unio nasutus. Say in Encycl. Am. Conch, pi. iv. Jig. 1. 



J. HIS is one of the most natural genera in the modern systems 

 of conchology, as it includes all fresh-water bivalves having two 

 rough cardinal teeth in one valve and one in the other. The 

 colours of all are more or less dark-brown, sometimes radiated 

 with green ; but the specific characters rest on the contour of the 

 ghell and the proportion of the teeth. 



There can be no doubt this shell is the Unio nasutus of Say, 

 who refers to the figure of Lister. The Unio nasuta however of 

 Lamarck I apprehend will be found different, as he seems to 

 think ; his shell also is purple inside with short thick teeth ; in- 

 deed so much uncertainty hangs on the shells of this genus, that 

 the species can only be fixed by ample descriptions and very 

 correct figures. The figure is from a shell in the Linnaean So- 

 ciety's cabinet. Mr. Say says it is common in the Delaware 

 River, North America. 



PI. 57. K 



