THE NAIADES OF MISSOURI lOl 



inner gills larger, much wider anteriorly, inner lamina free from 

 visceral mass nearly whole length; palpi not large, triangular, 

 united partly anterio-dorsad. 



Reproductive Structures: — Marsupium typically Anodon- 

 tine; glocliidium next to largest on record (0.380 x 0.390mm) 

 wider than long, hinge line undulate; soft parts yellow; marsu- 

 pium, however, rich brown when charged. 



SHELL CHARACTERS. 



External Structures: — Shell moderately large, thin to 

 moderately thick, elongate, compressed (especially in male), 

 posterior end bi-angulate, costae on slopes of post- dorsal ridge; 

 disk without sculpturing; umbones peculiarly marked with four 

 coarsely mingled concentric and double-looped bars; epidermis 

 from light horn-color to dark chestnut in old specimens, greenish 

 and rayed in young. 



Internal Structures: — Cardinals single in right, rather 

 tripartite in left valve and interdentum deeply notched; laterals 

 almost obliterated; nacre deep rich cream in umbonal cavity, 

 while lavender and b ue on pallial border, often whole nacreous 

 surface yellowish or ferruginous due to a distomid infection to 

 which this species is so susceptible. 



SHELL measurements. 



Sex Length Height Diameter Um. ra. Locality 



9 116 X 85 X 44mm 0.370 (Gascondy, Mo., Gasconade R.) 

 cf 125 X 56 X 35 " 0.280 (Black R., Williamsville, Mo.) 

 9 100 X 52 X 32 " 0.275 (Miss. R., LaGrange, Mo.) 

 75 X 42 X 24 " 0.280 (St. Francis R., Greenville) 



Miscellaneous Remarks: — 5. costaia is found in sandy and 

 muddy situations, is a deep burrower and very active. With this 

 habitat and physiological characters it is strange that it should 

 not be found at all in the sluggish muddy streams of North Mis- 

 souri and very rarely in Central Missouri where the streams are 

 intermediate Tor clearness and current. Perhaps this species has 

 the most variegated nacre of any Naiad shell. It is a very common 

 species in the clear and rapid streams of the Ozark Plateau and 

 Center where its shell is duller epidermis than those of the Miss- 

 issippi. {See PI. xxii, Figs, fi A — F .) This compression and plica- 

 tion are due, doubtless, to swifter current and more rocky bottom. 



