26 NAIADES OF MISSOURI 



Miscellaneous Remarks: — This species is distinguished 

 mostly from F. iindata and the intergradcs by its more quadrate 

 and compressed form of shell, by lower beaks, and less upright 

 position. F. undata trigonoid-cs is' separated from it by possessing 

 a heavier and more elongated shell with a more prominent post- 

 umbonal ridge and black epidermis. The White River shells, 

 indicated in the above measurements, are rather intergrades 

 for this species and Jwbetata and the vSt. Francis flava are too 

 inflated to be very typical. Since this vState proves to be such 

 grounds for the inconstant occurence of types, and this species 

 is so susceptible to intergradation, it is difficult to find a typical 

 flava, such as found in the Interior Basin east of the Mississippi. 

 Perhaps its nearest form is in drainage for the south slope of the 

 Ozarks in this state, although Simpson reports it as having a 

 general distribution throughout the Mississippi drainage. This 

 distribution doubtless included its many forms. Simpson further 

 states that the St. Lawrence River system includes y?a?)a. Dr. Sterki 

 (1898, p. 30) considers this species as occasionally hermaphroditic 

 by examination of its gonads. vSurely this finding can be confirmed 

 by the forms of Missouri, for it is rarely that it is even locally a gouo- 

 chorist. Flava is typically tachytictic being only found gravid 

 from May until August. 



Fusconaia hebetata (Conrad). 

 PL XV,— Figs. 33 A and B. 

 1854 — Unio hehelatus Conrad., Jl. Ac. N. Sci. Phila. II, p. 296, PI. 



XXVI, Fig. 5; 1888— B. H. Wright, Check List. 

 1900b — Quadrula hebetata Simpson, Proc. U. S. Nat. Mus., XXII, 



p. 787. 

 Animal 'Characters:- — The soft parts of a form of F. hebetata, 

 found in the Osage River, were discovered to be indentical with 

 those of F. flava. 



SHELL CHARACTERS. 



External Structures: — Shell rather orbiculate-quadrate, 

 thick, moderately inflated, post-umbonal ridge prominent, disk 

 smooth, beaks flat, and well back from the anterior end post- 

 dorsal ridge rounded, post-ventral margin gently undulate; 

 epidermis black with a few faint imbricated rays toward (but 

 not across) the disk in the middle of the umbonal region. 



Internal Structures:— Cardinals single in right, double 



