THE NAIADES OF MISSOURI I9I 



all over; nacre white; the antero-extra pallial layer remarkably 

 thickened; anterior muscle scars deep, the posterior lightly im- 

 pressed and confluent; the pallial line reflected upward and inward 

 in the post-half; cardinals double in each valve, small, high, 

 accuminate, sulcate; sexual expansion thin, swollen, slightly 

 radiately and concentrically ridged, denticulate on edge. 



Shell Characters of Male: — Shell much the smaller of 

 the two (so far as noticed) ; rounded before, sinuous below, widely 

 biangular behind, flattened over the umbones and to the post- 

 base; post ridge widely double. 



Beaks of both sexes remarkably heavily ridged, inclined to be 

 doubly looped, but obsately so in front, ridges heavy behind 

 running downwards and backwards to the umbonal ridge. The 

 earlier growth of the shell when looked at through a lens resembles 

 in a striking way a diminutive Amblema plicata (Say.) 



Sex Length Height Diameter Locality 



9 33 X 23 X 15 mm — (White R., HoUister, Mo.) 



cf 22.5 X 19.5 X 13.5mm — (White R., Holilster, Mo.) 



9 26.5 X 18.5 X 14 mm — (White R., Hollister, Mo.) 



Miscellaneous Remarks:- — The position of this interesting 

 species is exactl}^ between capsaejormis Lea and hiemargiatus 

 Lea. From capsaeformis our species differs in the sexual enlarge- 

 ment being of the same general body color and in being more 

 expanded or swollen in the middle and therefore not so regularly 

 fan-shaped as in capsaejormis and our shell is not so regularly 

 rounded behind. From hiemarginata our species may be differen- 

 tiated by its lack of pronounced angles and ridges of the former 

 and b}' our species being much smaller and thinner and from its 

 general almost solid piece color. Our species differs most remarkably 

 in the heavy beak sculpture. From deviatus Anthony our shell 

 similarly differs in color; and the sexual swelling is not so far 

 protruded behind. A specimen of deviatus, recently procured, 

 shows this species to have beaks nearly as heavily corrugate as 

 our species and not, as Anthony supposed, nearly smooth as in 

 capsaejormis. The presence of a form so intimately resembling 

 those of the East Tennessee mountain streams in Missouri is of 

 great interest. 



The shell was taken by the co-author, W. I. Utterback, in 

 the White River, Hollister, Mo., Aug. 26, 19 13, and is named in 

 honor of Dr. W. C. Curtis of the Department of Zoology, Univer- 



