10 Trans. Acad. Sci. of St. Louis. 



Var. testa versus extremitatem lateris anlici serhsim depressa , 

 magis attenuala. 



Unio modioUformis Say, Anier. Conch. 



Habite dans le lac Erie. Michaud fiJs. * * * Test 

 tres blanc. Longueur apparente 72 miUirnetres. La variete 

 b fit dans la riviere de la Nouvelle-Ecosse. * * * Lon- 

 gueur apparente 53 millimetres.^^ 



Lamarck's brief description is no doubt responsible for 

 very much of the confusion which attends the separation of 

 this form. To aid in its correct determination the following 

 description has been drawn, from specimens collected in the 

 Duck river, Tennessee. The specimen figured is one of those 

 employed in this diagnosis. 



Shell smooth, somewhat elliptical, most of its mass posterior 

 to a line drawn vertically from the umbones, laterally sub- 

 compressed, somewhat pointed posteriorly, circularly rounded 

 before ; umbones prominent and pointed anteriorly, apic- 

 ulate; ligament large, thick, light brown; epidermis honey- 

 yellow, smooth, often polished, rayed from the tips of 

 the umbones with green lines over the first formed half of 

 the disk, the rays broadening downward, occasionally in- 

 terrupted by the lines of growth which are numerous and 

 strongly impressed giving to the lower third of old specimens 

 a striate appearance; umbonal slope rounded anteriorly but 

 compressed posteriorly; in the female, the posterior slope is 

 rather less flattened than in the male, while the outline of the 

 disk is less pointed ; cardinal teeth single in the right, double 

 in the left valve, rather short and incrassate, crenulate ; plate 

 connecting laterals with the cardinal teeth disposed to folding 

 in the left and pitted in the right to correspond, thick, slightly 

 arched; lateral teeth long-lamellar, curved ventrad, striate; 

 anterior cicatrices confluent, deeply impressed, that of the 

 retractor pedis impression very deep and circular and at the 

 end and lower margin of the lateral teeth and partly on them ; 

 dorsal cicatrices numerous, small and impressed in an irregular 

 line on the under side of the plate between the cardinal and 

 lateral teeth ; nacre pure white, with a very marked iridescence 

 posteriorly between the pallial cicatrix and the margin. 



Animal not observed. 



