THE NAIADES OF MISSOURI IO3 



Arcidens confragosus (Say). 



("Black Pocket-Book," "Black Pocket," "Rock Shell," 

 "Rock Pocket-Book.") 



PI. XXII, Figs. 72 A and B. 



1829 — Alasmodonta confragosus Say, N. Harm. Dis., II, p. 339. 

 1888 — Margaritana confragosa B. H. Wright, Check List. 

 1900b — Arcidens confragosus (Say) Simpson, Proc. U. S. Nat. Mus. 

 XXII, p. 662. 



ANIMAL CHARACTERS. 



Nutritive Structures: — ^Branchial opening large, with few 

 papillae; anal with tiny papillae on inner edge; supra-anal very 

 long, connected antero-dorsad two-thirds of length; inner gills 

 somewhat wider, inner laminae free from visceral mass; color 

 of gills and palpi brown, all other soft parts soiled white and chamois 

 like. 



Reproductive Structures: — -Marsupium occupying only 

 outer gills, light brown when sterile, spotted and vertically striated 

 when gravid with early embryos, padiform and dark brown when 

 gravid with ripe glochidia; internal structure of gravid marsu- 

 pium typically Anodontine; glochidium of specimen from Platte 

 River, Missouri, (0.355 ^ 0.350mm.) but possessing same shape, 

 i. e., subtriangular, with undulate hinge line. 



SHELL CHARACTERS. 



External Structures : — Shell subrhomboid, inflated, roughly 

 sculptured with apiculated tubercles in umbonal region, ribbed 

 tubercles on post-umbonal ridge, coarse undulations across post- 

 ventrad part of disk, slopes of post-dorsal ridge sub-costated ; 

 beaks full, high, corrugated; epidermis brown to black in adult, 

 greenish mingled with black in youth. 



Internal Structures: — ^Cardinals single in right valve, 

 double in left, the posterior one being long, serrated and placed 

 just under the beak in place of the interdentum; laterals, faint in 

 both vales; scars rather well impressed; umbonal cavity somewhat 

 deep; nacre bluish to white with sky-blue border. 

 Sex Length Width Diameter Um. ra Locality 



9' 103 X 67 X 48 mm 0.30 (Platte R., Mo.) 



102 X 65 X 48 " 0.34 (L. Contrary, Mo.) 



c?" 115 X 74.5 X 51.5 " 0.31 (Osage R., Mo.) 



45 X 33 X 22.5 " 0.30 ( " " " ) 



