NAIADES OF MISSOURI 8s 



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 X 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 tl^e rounded and compressed 

 posterior half of the latter. The shell of missourense compares 

 well with that of Fusconaia suhrotunda 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 (Lea).' 



' More recently (April, 1915) Mr. Walker determines this species as 

 a Quadrula of the subrotunda groupp (Nautilus XXVIII, PI. V., figs, i and 2). 



