d THE NAIADES OF MISSOURI 253 
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. Frierson 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 Alasmidonta 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 Jacustrine 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. 
