252 THE AMERICAN MIDLAND NATURALIST 
inner gills larger, much wider anteriorly, inner lamina free from 
visceral mass nearly whole length; palpi not large, triangular, 
united partly anterio-dorsad. 
REPRODUCTIVE STRUCTURES:—Marsupium typically Anodon- 
tine; glochidium next to largest on record (0.380 x 0.390mm) 
wider than long, hinge line undulate; soft parts yellow; marsu- 
pium, however, rich brown when charged. 
SHELL CHARACTERS. 
EXTERNAL STRUCTURES:—Shell moderately large, thin to 
moderately thick, elongate, compressed (especially in male), 
posterior end bi-angulate, costae on slopes of post-dorsal ridge; 
disk without sculpturing; umbones peculiarly marked with four 
coarsely mingled concentric and double-looped bars; epidermis 
from light horn-color to dark chestnut in old specimens, greenish 
and rayed in young. 
INTERNAL STRUCTURES:—Cardinals single in right, rather 
tripartite in left valve and interdentum deeply notched; laterals 
almost obliterated; nacre deep rich cream in umbonal cavity, 
while lavender and b'ue on pallial border, often whole nacreous 
surface yellowish or ferruginous due to a distomid infection to 
which this species is so susceptible. 
SHELL MEASUREMENTS. 
Sex Length Height Diameter Um. ra. Locality 
9 116 x 85 x 44mm 0.370 (Gascondy, Mo., Gasconade R.) 
mL 2G Ie kee SOL MKS S'S 0.280 (Black R., Williamsville, Mo.) 
OR TOON ER 52) eX) 132k 0.275 (Miss. R., LaGrange, Mo.) 
7a X apex 24-2? 9.280. .(St: Hrancis- RB. Greenvale) 
MISCELLANEOUS REMARKS:—S. costata is found in sandy and 
muddy situations, is a deep burrower and very active. With this 
habitat and physiological characters it is strange that it should 
not be found at all in the sluggish muddy streams of North Mis- 
souri and very rarely in Central Missouri where the streams are 
intermediate for clearness and current. Perhaps this species has 
the most variegated nacre of any Nazad shell. It is a very common 
species in the clear and rapid streams of the Ozark Plateau and 
Center where its shell is duller epidermis than those of the Miss- 
issippi. (See Pl. xxii, Figs.71 A—F.) ‘This compression and plica- 
tion are due, doubtless, to swifter current and more rocky bottom. 
