THE NAIADES OF MISSOURI 99 



any of the Anodontinae on account of its sculpturing on the pos- 

 terior dorsal ridge, the costae there being somewhat similar to 

 those on the shell of some Amblemae or Quadrulae. Simpson 

 treated this genus under three sub-genera and while the shell 

 characters may greatly differ, yet, the animal characters are so 

 constant and the known species are so few to retain this subgeneric 

 treatment. The type, S. compressa (Lea), is not found in this 

 state — ^not even in the Mississippi River. Only two species of 

 this genus, 5. complanaia (Barnes) and costata (Rafinesque), are 

 found in Missouri and they are .not widely distributed, the former 

 being confined mostly to the north and the latter to the south 

 part of the state. 



Symphynota complanata (Barnes). 



("Heel Spitter," "Hackle Back," "Hatchet Back," "Pan-cake.") 



PL XXII, Figs. 70 A and B. 



1823 — Alasmodonta complanata Barnes, Amer. Jour. Sci. and Arts, 



p. 278, pi. XIII, fig. 21. 

 1900b — Symphynota complanata (Batnes) Simpson, Proc. U. S. Nat. 



Mus., XXII, pp. 665-666. 



ANIMAL CHARACTERS. 



Nutritive Structures: — Branchial opening thickly set 

 with short papillae; anal finely dentate; anus of intestine fringed 

 with three or four papillae; supra-anal large, mantle connection 

 widely separating it from anal; inner gills, wider than outer, 

 inner laminae free except briefly, anteriorly; marsupia occupy 

 entire outer gills, truncated along ventral margin when charged, 

 ovisacs not divided ; glochidium very large, spined, hinge line 

 undulate measures 0.310 x 0.320mm; most of soft parts yellowish, 

 gills rusty color when gravid. 



SHELL CHARACTERS. 



External Structures: — Usually rhomboidal, compressed; 

 alae high, marked with a few upcurved costae; disk smooth; 

 beaks low, sculptured with coarse double-looped ridges; epidermis 

 of young shell reddish with brown rays; of old specimens, brown 

 to black. 



Internal Structures: — Cardinals single in right, double in 

 left valve; laterals faintly double in both valves; umbonal cavity 

 shallow; nacre bluish to satiny white with marginal ribbon of 



