THE NAIADES OF MISSOURI 197 
Pleurobema Utterbackii Frierson. New Species. 
Pl. V., Figs. 12a and b; Pl. XX., Figs. 63 A —D. 
ANIMAL CHARACTERS. 
“NUTRITIVE STRUCTURES:—Branchial opening with many 
short antennae; anal very finely crenulated; 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. 
SHELL 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 Pd en 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 
@ 68 x 42 x 26 mm (White River, Hollister) 
CUA OS NeXT MG KN aise h (23 (Jack’s Fork of Current R.) 
ile iitelaGy (oss amaelitD:< 7a Bens (Chants 4 a ” tes) 
OVMET SOM Re eTE eon sO} es (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. 
