THE NAIADES OF MISSOURI 189 
1912b—Pleurobema pyramidatum (Lea) Ortmann, An. Car. Mus., 
VIII, p. 264. 
ANIMAL CHARACTERS. 
NUTRITIVE STRUCTURES.:—Branchial opening densely papil- 
lose; anal finely papillose; anal and supra-anal very closely 
connected by mantle edges—sometimes connection deciduous ;— 
inner gills broader, longer, inner laminae free from visceral mass; 
palpi long and thickened; most of soft parts dirty white, mantle 
edges at branchial openings black. 
REPRODUCTIVE STRUCTURES :—Only outer gills marsupial; 
sterile marsupia with crowded septa, those of male gills very 
distinct and more separated; no gravid specimens found. 
SHELL CHARACTERS. 
EXTERNAL STRUCTURES:—Shell obliquely pyramidal or trap- 
ezoidal, very solid and heavy anteriorly; disk smooth; beaks 
very full and projecting anteriorly; rather straight dorsad, greatly 
curved ventrad, epidermis black. 
INTERNAL STRUCTURES:—Cardinals single in right, double in 
left valve; laterals double in left, single in right; scars deeply 
impressed; beak cavities moderately deep; nacre white, tinged with 
blue posteriorly—sometimes pinkish. 
Sex Length Height Diameter Locality 
He OL Se Oe ee Pivosrehen (Osage R., Sagrada) 
OR USES ke 5225 Xe AA (Cee ” Warsaw) 
OUI a or ke 54 Xa Ors (Meramec R., Fern Glen) 
He 27.5 Me2Os5 Ki LOS iiim-s (Osage Ry —Baker) 
Juvenile shell thick, almost globular, very smooth; beaks 
full but not protruding anteriorly, sculptured with two or three 
ridges arranged rather concentrically and breaking into three 
coarse tubercles at base of post-umbonal ridge; epidermis reddish 
and leather-like with rays on the anterior half of shell; lateral 
teeth more inclined to double in right valve than in mature shell; 
beak cavities very shallow; nacre solid pink. 
MISCELLANEOUS REMARKS:—This pyramidatum is the same 
as found in Arkansas and Oklahoma where it is also found unasso- 
ciated with typical obliquum. The species, Pleu. pyramidatum 
(Lea) and obliquum (1am.) are most typically represented in the 
Tennessee—Cumberland system and the fact of their forms turning 
up in the South West (i. e., in the region south of the Missouri and 
