454 THE AMERICAN MIDLAND NATURALIST 
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 
QO 33. xX 23° .-xX. 15° mm—(Wihite R. Hollister, Moy 
Oo 22.5 kX 19.5 X 13.5mm—(White R., Holilster, Mo.) 
Q 26:5 x 18.5 x 14 mm—(White R., Hollister, Mo.) 
MISCELLANEOUS REMARKS:—The position of this interesting 
species is exactly between capsaeformis Lea and biemargiatus 
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 capsaeformis and our shell is not so regularly 
rounded behind. From biemarginata our species may be differen- 
tiated by its lack of pronounced angles and ridges of the former 
and by 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 
capsaeformis. 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, 1913, and is named in 
honor of Dr. W. C. Curtis of the Department of Zoology, Univer- 
