THE NAIADES OF MISSOURI 1 37 



only represented in this State by ellipsis and that in a limited 

 distribution both in the geographical and individualistic sense. 



Obovaria (Pseudoon) ellipsis (Lea). 



("Missouri Nigger-Head," "Egg Shell," "Hickory Nut.") 



PL XXV, Figs. 84 A and B. 



1S28— Unto ellipsis Lea, Tr. Am. Phil. Soc, III, p. 268, pi. IV, fig. 4. 

 1900b — Obovaria ellipsis (Lea) Simpson, Proc. U. S. Nat. Mus.; 1912b, 

 Ortmann, Car. Mus., VIII, pp. 323-324. 



ANIMAL CHARACTERS. 



Nutritive Structures: — Branchial opening small with 

 two-ranked papillae; anal finely papillose; supra-anal crenulated 

 (a rare character) not well connected to anal; mantle border 

 just antero-ventrad to branchial opening with crowded papillae 

 or crenulations extending one-third of the way towards center 

 of ventral margin; gills very wide, both blunt and pointed poster- 

 iorly, inner laminae of inner gills entirely connected to visceral 

 mass; palpi rather small, connected about one-half of their length 

 antero-dorsad ; foot pinkish, mantle edge dark in region of siphonal 

 openings, rest of soft parts dirty white. 



Reproductive Structures: — Marsupium occupying pos- 

 terior portion of outer gills, rather kidney-shaped, consisting of 

 several ovisacs, twenty-five or thirty which, when gravid, extend 

 far below the original edge, pigmented purplish ventrad; conglu- 

 tinates white, not well formed, ova and glochidia discharged in 

 rather loose masses; glochidium medium in size, semi-elHptic, 

 rather short and straight, measures 0.210 x 0.265mm. 



SHELL structures. 



External Structures: — Shell sub-elliptical, rather oblique, 

 very solid, thick, heavy; ventral margin more rounded than 

 dorsal; disk smooth; no post-umbonal ridge; rest lines of growth 

 very distinct; beaks projecting anteriorly, rather prominent, 

 sculpturing indistinct; epidermis yellowish-horn color with green 

 rays in young shell. 



Internal StrucTur-ES: — Cardinals very heavy, nearly par- 

 allel to laterals with right post-cardinal converged dorsad and 

 rounded up from a broad V-shaped gutter; interdentum broad, 

 thick, right deeply gashed; beak cavities not very deep; nacre 

 white. 



