84 NAIADES OF MISSOURI 



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 



9 60 X 40 X 24 (White R., HoUister) 



cf 75 X 56 X 31 (St. Francis, Greenville) 



9 47 X 40 X 22 (White R., HoUister) 



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 (Plen. coccineuni) 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. 



\()Oi^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. 



