^02 THE NAIADES OF MISSOURI 



Its general distribution is for the St. Lawrence drainage and in the 

 Mississippi as far south as Texas. This species can be easily 

 identified and distinguished from other shells by rugose or ribbed 

 structure on the abrupt slopes of the post-dorsal ridge, by its 

 peculiar deep socket just underneath the beak, by its compara- 

 tively unridged laterals, but, most of all, by its very characteristic 

 compound beak sculpturing. Because of these very striking shell 

 characters it may deserve the subgeneric treatment of Simpson 

 (1900b p. 664) who gave this species the name Symphynota {Las- 

 migona) costata (Raf.) Dr. Ortmann and Mr. Frier son think this 

 subgenus, Lasmigona, really deserves generic rank. The author 

 has found the breeding season of costata to extend from August 

 until May with eggs and early embryos for late summer and fall 

 and glochidia for winter and spring. 



Genus Arcidens Simpson 



1900b — Arcidens Simpson, Proc. U. S. Nat. Mus., XXII, p. 661. 



(Type Alasmidonia confragosa Say). 



ANIMAL CHARACTERS. 



Branchial opening densely set with papillae; anal finely 

 serrated; supra-anal long with short mantle connection to anal; 

 inner gills wider in front than outer, inner laminae of inner gills 

 free; palpi large, united two-thirds of their length antero-dorsad; 

 marsupium Anodontine both in external and internal structures; 

 glochidium spined, large, hinge line undulate. 



SHELL CHARACTERS. 



Shell somewhat rhomboidal, inflated with rather high full 

 beaks; disk and beaks profusely sculptured, the latter coarsely 

 double-looped, the spinuous tuberculed loopes extending in two 

 diverging rows upon the disk; the former with oblique folds on the 

 post-ventrad part with pustulated expansions along the post- 

 umbonal ridge; cardinals present but only traces of laterals are 

 seen; nacre white. 



This genus is represented in this state by few individuals 

 and while the only species of this genus, known so far, is both 

 fluviatile and lacustrine it is more often found in quiet creeks, 

 head waters of rivers or in other lacustrine conditions of the rivers, 

 such as the pond-like stretches, sloughs, bayous, etc. 



