THE NAIADES OF MISSOURI 439 
glochidia rather large, semi-elliptical, spineless, regularly rounded 
ventrad, hinge-line straight, measures 0.270 x 0.330 mm.; mantle 
edge antero-ventrad to branchial opening in female with numerous 
regular papillae extending quite down to the central part of ventral 
border. 
SHELL CHARACTERS. 
EXTERNAL STRUCTURES:—Shell small to medium in size, 
rather elongate-elliptic, thin, compressed; post-umbonal ridge 
rounded; female shell very wide and blunt posteriorly, male 
pointed and narrow; umbones low, sculptured by eight or ten 
coarse, regular, inverted V-shaped ridges with the apices pointed 
toward tips of beaks, the posterior ridges longer and more disposed 
obliquely across base of post-ridge; disk without sculpturing; 
epidermis brown to black with many rays disposed posteriorly 
and showing through on the nacreous surface. 
INTERNAL STRUCTURES:—Cardinals double in both valves, 
rather thin and erect; laterals thin, double in both valves; scars 
rather well impressed; beak and branchial cavities rather deep 
and hollowed out; nacre white to light bluish irridescent posteriorly. 
Sex Length Height Diameter Locality 
o 62 x 27 x =21.5mm—(Flat Creek, Sedalia). 
9 65 x 36 x 25.5mm—(Hinkston Creek, Columbia) 
o 50 x 28 x 15.5mm—(Lost Creek, Amity) 
Q@ 36 x 18 x 13.5mm—(Flat Creek, Sedalia) 
This last measurement is for one that has preserved soft parts 
and although it is very young and small yet it is gravid with normal 
glochidia. Its beak sculpture is very distinct as shown above in 
the description of shell character. 
MISCELLANEOUS REMARKS:—Subrostrata is a creek and pond 
shell, but in spite of this lacustrine disposition it is never found in 
any of the North West Missouri lakes. Like U. tetralasma it 
adjusts itself easily and quickly to artificial ponds and channels. 
It is never found in large rivers nor swift streams. It has a general 
distribution over the State, especially in the ponds and quiet 
creeks of Central part. Simpson gives it a general distribution over 
the entire Mississippi drainage north of about latitude 41°. The 
breeding season of subrostrata is a long one. Its glochidia seem to 
be very constant in size for widely separated localities. Compari- 
sons have been made of glochidia from mussels of Central Missouri 
