THE NAIADES OF MISSOURI III 



the main body of rivers but in such cases there was always some 

 slough, bayou, or lake near by from which its light shell may have 

 been carried over in time of flood. This species has a general 

 distribution over the Mississippi and St. Lawrence basins. In 

 this state it is confined to the lake district of N. W. Missouri, 

 and in the lacustrine conditions of Central and South West Mis- 

 souri. Its breeding season seems to be continuous for the year, 

 or at least there is a very short interim of sterility. The author 

 examined it nearly every month of the year to find it gravid and 

 that, for the most part, with mature (active) glochidia. The 

 "eye spots," mentioned by Simpson, as characteristic marks on 

 the mantle edge at the branchial opening, have not been observed 

 by the writer. Because of its Anodonta-like marsupia, but more 

 on account of its physiological characters, in being a hermaphrodite 

 with an almost continuous breeding season, this species should be 

 assigned to a little higher position in the genus than suborbiculata. The 

 fact that Rafinesque used ohiensis as the type for his genus Lastena 

 and also because of such departure in anatomical and concholo- 

 gical features from those of the genus Anodonta for ohiensis and 

 its nearest ally, suborbiculata, this genus Lastena should now be 

 employed for these two species of this State. 



Genus, Anodonta Lamarck. 

 1799 — Anodonta Lamarck, Prodrome Class. Coq., p. 87. 

 1 81 7 — Anodontes Cuvier, Regne. An., II, p. 472. 



(Type, Mytilus cygneus Linnaeus). 



Animal Characters :— Branchial opening with yellowish 

 papillae, anal smooth to slightly crennulated; supra-anal generally 

 small, removed from anal by long mantle connection ; inner lamina 

 of inner gills free from one-half to entire length; palpi usually long 

 and large; only outer gills marsupial, when marsupia are gravid, 

 ventral edge distends and secondary water-tubes appear, ovisacs 

 simple, undivided, dark brown when gravid with mature glochidia; 

 no conglutinates formed; glochidia large, brownish, spined, 

 spadiform. 



Shell CHARAG;rERS: — Shell elliptical, inflated, thin, slightly 

 alated; disk smooth; beaks full high, sculpturing distinct, double- 

 looped, angled upward centrally; epidermis polished, brightly 

 colored; hinge teeth completely lacking. 



No genus is so susceptible to so many mutations, 3^et it is 



