NAYADES. 99 



ORDER DIMYAIRA. 



Shell provided with two separate lateral muscular impres- 

 sions. 



SuB-DivisioN I.— LAMELLIPEDES. 



Family I.— NAYADES. 



Shells inhabiting fresh waters ; the hinge sometimes with an 

 irregular, simple, or complex tooth, and a longitudinal prolonged 

 one, and sometimes none. Some have a compound muscular 

 impression ; and the umbo is frequently decorticated. 



Genus I ANODON Bruguiere. 



Shell equivalve, inequilateral, and transverse, for the most 

 part very thin ; hinge line nearly straight ; hinge glabrous, with 

 smooth lamina, destitute of cardinal teeth, truncated, or form- 

 ing a sinus at the anterior end, terminating the apex of the 

 shell ; two lateral remote, muscular impressions, the posterior 

 one being compound ; pallial impression entire, and seldom dis- 

 tinctly marked ; ligament linear, external, sunk in a cleft at the 

 anterior extremity ; inside pearlaceous ; external surface covered 

 with a shining epidermis. 



1. Anodon Cygneus. 



Anodonta Cygnea, Lamarck, An. San. Vert., VI, p. 84; Dra- 

 pernaud, Hist. Nat. des Moll., p. 134, pi. 11, f. 6, and pi. 12, f. 

 1; Anodon Cijgneus, Turton, Brit. Biv., p. 239; MytUus Cyg- 

 neus, Linnaeus, Syst. Nat., p. 1158; Gmelin, p. 3355; Montagu, 

 Test. Brit., p. 170. 



Shell thin, oblong-ovate, tumid; anterior side short, rounded; 

 posterior slope elongated, and somewhat pointed, more or less 

 angulated, and subcompressed ; umbones but slightly produced, 

 and placed nearest the anterior side. 



This species is subject to great variety of form. These 

 varieties have, by different authors, been considered distinct 

 species ; but we have not been able to detect any specific dif- 

 ference in the animal inhabitant. There are, however, several 

 well-Hiarked and permanent varieties, the distinguishing charac- 



