196 THE AMERICAN MIDLAND NATURALIST 
SHELL CHARACTERS. 
Through the kindness of Mr. L. S. Frierson the writer was 
loaned a shell of this rare species from the original lot, described 
by Mr. Marsh and collected by Mr. Elwood Pleas in the Black 
River, Popular Bluff, Butler County, Missouri. From the fact 
that the writer had but one shell before him, he quotes a more 
complete description from the author than can be given without 
a series at hand:—‘‘Shell smooth, obliquely triangular, rounded 
behind, subbiangular behind, moderately thick, very ‘much 
thicker anteriorly, sides somewhat flattened, beaks wide, solid, 
incurved; ligament long, light brown; epidermis light brown, 
without rays, growth lines numerous, not raised; umbonal slope 
wide and rather flat; posterior slope wide, flattened with two 
dark inconspicuous lines running from beaks to posterior margin; 
beak sculpture unknown; cardinal teeth rather long and solid, 
depressed, disposed to be double in both valves, corrugate; lateral 
teeth straight, oblique, corrugate; anterior cicatrices distinct, 
deep, post cicatrices distinct and well impressed; shell cavity 
wide and deep; nacre white.” 
The above description was made from four specimens from 
young to adult. The young shells are much flatter. The measure- 
ment of one of these adult shells is: 
Length Width Diameter Locality 
66 x 54 xX 36mm (Black R., Popular Bluff) 
MISCELLANEOUS REMARKS:—The author of this species states 
that he knows of no described species which closely resembles it. 
From the single specimen in hand it seems to be rather intermediate 
for P. catillus and coccineum; however, it does not possess the 
radial furrow of the former nor the rounded and compressed 
posterior half of the latter. The shell of missourense compares 
well with that of Fusconaia subrotunda and if its soft parts could be 
obtained for study it might be found to be a form of the latter 
as it is often difficult to separate the species of Fusconaia and 
Pleurobema solely on the basis of shell characters. Even from 
character of shell, Frierson would group this species under Fus- 
conaia; however, Walker, who considers this a valid species, 
would class it near P. estabrookianum (1ea).’ 
' More recently (April, 1915) Mr. Walker determines this species as 
a Quadrula of the subrotunda groupp (NautilussX XVIII, Pl. V., figs. 1 and 2). 
