THE NAIADES OF MISSOURI 195 
three knotty elevations behind; epidermis marked by darker 
bands parallel to the growth lines, rayed in young. 
INTERNAL STRUCTURES:—Cardinals sharply doubled in both 
valves; interdentum cut away in the left valve to receive the 
posterior left cardinal; laterals curved, double in left single in 
right; umbonal cavity moderately deep; nacre salmon pink to 
white. 
Sex Length Height Diameter Locality 
P60, ok AOS Xia (White R., Hollister) 
Ca 78) eke ON oe nnaiy (St. Francis, Greenville) 
G uediy oe wal saw ek! 122 (White R., Hollister) 
Note that the younger shell is more rounded and squarer, 
has more of an olivaceous epidermis with green rays; umbonal 
sculpture, plainer with three bumps on base of post-ridge; nacre 
more pearl bluish. 
MISCELLANEOUS REMARKS:—This species (Pleu. coccineum) is 
most typically represented in the South and South-west Missouri 
drainage systems and, while it is not found at all in North Missouri. 
it is only represented occasionally by mere forms in Central Mis- 
souri. It is usually restricted to the smaller streams or to the 
headwaters of large rivers. Normally, marsupial characters are 
limited to the outer gills only, although this fact has been denied 
by some. The writer on one occasion, found this species with all 
four gills gravid, but in all other cases the outer gills were only 
found functioning as marsupia. In this species and in some other 
Pleurobemae, there may be some variabilty in this respect. Coc- 
cineum has a short breeding season as determined by the writer’s 
breeding record. 
Pleurobema missourense (Marsh). 
(‘‘Missouri Shell.’’) 
Not figured. 
1901—Pleurobema Missouriensis Marsh, Nautilus, XV, pp. 74-75. 
ANIMAL CHARACTERS. 
Not having seen the soft parts of this species the writer can 
offer no description. Although the author of missourense gave no 
such description, yet the establishment of this species within 
the genus Pleurobema must have been inferred from these char- 
acters. 
