86 NAIADES OF MISSOURI 



Pleurobema Utterbackii Frierson.' New Species. 

 PL v., Figs. 12 a and b; PL XX., Figs. 63 A ~D. 



ANIMAL CHARACTERS. 



"Nutritive Structures: — Branchial opening with many 

 short antennae; anal very finely cr'enulated; supra-anal with 

 short but distinct mantle connection to anal; gills long, much 

 pointed posteriorly, inner laminae of inner gills free from visceral 

 mass; palpi wide, short connected about two-thirds of their length 

 antero-dorsad ; color of soft parts mostly a light tan, mantle 

 edges at siphonal openings blackish, gills of male and sterile female 

 a darker tan. 



"Reproductive Structures: — No gravid females found, 

 but sterile ones only present outer gills as marsupial; sterile 

 marsupia wider with more crowded septa than outer gills of male. 



SHEiviy characters. 



' ' External Structures : — Shell elliptical, somewhat rhom- 

 boidal; evenly truncate above posteriorly; beaks at one fourth 

 of their length; epidermis rough, dark reddish brown, faintly 

 rayed when young; post-slope somewhat biangular and low; 

 beaks rather strongly corrugate. 



"Internal Structures: — Cardinals teeth roughish, inclined 

 to be double in both valves; laterals long, medium size; inter- 

 dentum narrow; muscle scars well marked and separate; cavity 

 of shell irregular, that of the beaks of medium depth; nacre white, 

 sometimes pinkish, irridescent behind. 



Sex Length Height Diameter Locality 



9 68 X 42 X 26 mm (White River, HoUister) 



c? 65 X 41 X 25.5 " (Jack's Fork of Current R.) 



c? 18.5 X 12.5 X 7.5 " ( " " " " " ) 



9 18.0 X 12.5 X 7.0 " (White R., Hollister) 



"Two juvenile shells are at hand measuring as above. Epider- 

 mis of latter olive-green, of the former, yellowish, both with 

 fine rays; nacre of latter bluish, of the former pinkish; beak 

 sculpture of both roughly corrugate, the three or four coarse ridges 

 curved up posteriorly into hummocks and directing the apices 



^ This description of Pleu. Utterbackii is quoted from the M. S. of 

 Mr. L. S. Frierson and is kindly permitted to be published here. 



