176 THE NAIADES OF MISSOURI 



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 



cf 62 X 27 X 21.5mm — (Flat Creek, Sedalia). 



9 65 X 36 X 25.5mm — (Hinkston Creek, Columbia) 



cf 50 X 28 X 15.5mm — (Lost Creek, Amity) 



9 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 3^oung 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. tetralasnia 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 



