NAIADES OF MISSOURI 75 



double in right; umbonal cavity moderately deep; nacre pearly 



white, irridescent. 



Sex Length Height Diameter Um. ra. Locality 



9 87 X 70 X 47 — 0.170 (Miss. R., Hannibal) 



cf 63 X 45 X 34 — 0.230 (Des Moines R., Dumas) 



9 55 X 40 X 32 — 00.180 (Little Blue, Courtney) 



cT 33 X 22 X 20 — 0.210 (Des Moines R., Dumas) 



This last measurement of a young shell shows great inflation 

 comparatively. (See Figs. 56 C and D.) 



Miscellaneous Remarks: — This species has a general 

 distribution over the Ohio - Cumberland system. It is not 

 uncommon in the Mississippi and Des Moines Rivers, but has 

 only been found in one interior stream, the Little Blue, Kansas 

 City, where it was collected by Mr. Bush and donated to the U. S. 

 National Museum under No. 134,642. Some have found all 

 four gills of aesopus marsupial, but most observations seem to prove 

 that only the outer are used as marsupia. The writer has only 

 observed outer gills as marsupial — even in case of many indivi- 

 duals with sterile gills. He is able to verify Dr. Ortmann's obser- 

 vation of charged marsupia with the "lilac hue." Its accidental 

 host has been found to be "sauger" (S. canadense). 



Genus Pleurobema Rafinesque. 



1820 — Pleurobema Rafinesque, Monograph of Bivalve Shells of River 



Ohio, Ann. Gen. Sci. Phys. Brux., p. 313. 

 1900b — Pleurobema (Raf.) Simpson, Proc. U. S. Nat. Mus., XXII, 



p. 745 (amended). 



(Type, Unio clava Lamarck.) 



Animal Characters: — Anal opening with short mantle 

 connection to supra-anal; inner gills much longer, inner laminae 

 free from visceral mass; palpi small very pointed; only outer 

 gills marsupial; ovisacs distend but little when gravid; conglutin- 

 ates white, narrowly leaf -like or lanceolate, not broken ; glochidium 

 small, spineless, subovate. 



Shell Characters: — Shell subtrapezoidal, subquadrate, 

 rounded or elongated, upright, or, when oblique, with beaks 

 produced anteriorly; beaks usually rather full and high, sculptured 

 obscurely with concentric ridges not extending out on the disk; 

 disk without sculpture; epidermis olivaceous, reddish brown or 

 even, black, rays more or less present in umbonal region; hinge 



