l^-> 



NAIADES OF MISSOURI 8 1 



ANIMAL CHARACTERS. 



Nutritive Characters: — Branchial opening with two ranks 

 of papillae; anal crowded with fine, short papillae; supra-anal 

 moderately separated from anal; inner gills slightly wider than 

 outer, inner laminae free from visceral mass; palpi thick and long; 

 connected antero-dorsad over half of length; color of soft parts 

 dingy white, for most part. 



Reproductive Structures: — Only outer gills marsupial; 

 when gravid marsupia cream colored, somewhat padiform ; conglu- 

 tinates white, leaf-like, solid; glochidium medium, subovate 

 spineless, measures 0.150 x 0.155mm. 



SHELL CHARACTERS. 



External Structures: — Shell roundly elliptical, thick and 

 solid anteriorly, moderately thin and compressed for posterior 

 half; post umbonal ridge flattened; beaks not prominent, nor 

 protruding anteriorly, sculptured by concentric corrugations; 

 epidermis reddish brown, rayed anteriorly. 



Internal Structures: — Cardinals double in both valves; 

 laterals single in right, double in left valve; interdentum wide and 

 thin; beak cavities shallow; nacre rose pink. 



Sex Length Height Diameter Locality 



c? 74 X 62 X 34 (Osage R., Warsaw) 



9 64 X 54 X 31 ( " " " ) 



cf 40 X 37 X 22 ( " " Baker) 



Miscellaneous Remarks: — No juveniles of this form are 

 at hand for description. The writer met with this variety in the 

 Osage. It can be easily distinguished from the more typical cocci- 

 neum of South Missouri in not possessing such compression for 

 the posterior half of shell, in being more elliptical and heavier. 

 The general outline is that of 0. ellipsis. It can be separated from 

 Pleii. ohliquuni catillus by this elliptical shape, but its greater 

 compression (especially posteriorly) and by its thinner, less solid 

 shell. The writer made an interesting discovery while examining 

 a gravid individual of this form afield with a (X12) lens to find its 

 late embryos in rapid rotary motion around one axis. This phe- 

 nomenon had been observed b}^ the author in Lastena ohiensis. 

 A breeding record of this form shows it to be a summer breeder 



