The latter measurement is that of a juvenile taken in a cutoff 
slough at Warsaw, Mo. Like that of other juveniles of this species, 
its supra-anal opening is found to be rudimentary—a mere furrow— 
and the gills are specked with minute black pigmented spots. 
Its shell is more rhomboidal than older; also two rows (five in a 
row) of ribbed tubercles; more prominent, coarse undulations not 
so distinct, nor horozontal; more zigzag sculpturing on disk; 
epidermis more blue-greenish; alae more costated; nacre more 
irridescent and bluish in beak cavities and with a brighter lavender 
ribbon around the margins. 
MISCELLANEOUS REMARKS:—Arc. confragosus is peculiar in 
its shell structure by its profuse subspiny sculpturing on disk and 
beaks, its coarse tubercles—especially on umbonal ridge being 
smooth dorsad and ribbed ventrad and by its postero-cardinal of 
left valve being long, coarsely serrate and in place of interdentum. 
In many characters this shell is like that of of Arkansia wheeleri 
W. and-O., but differs in not having well developed laterals and 
in having more profuse and prominent sculpturing. Confragosus is 
fond of quiet water and muddy bottoms; thus it is more lacustrine 
and when fluviatile it is found in creeks or in the head waters and 
bayous of the large streams. It has a general distribution from 
western Indiana to Iowa. Simpson reports it as most abundant in 
Illinois and is also generally found in the Mississippi and in the 
states adjoining, although it is, by no means, a common shell 
anywhere. It is a rare species even for North and Central Missouri 
where there are more lacustrine conditions and is not found at all 
in South Missouri. The author has found it gravid with active 
glochidia the latter part of January and with late embryos in the 
middle of March and great numbers were examined daily during 
July and August to find it sterile; thus it is bradytictic. 
Genus, Lastena Rafinesque. 
1820—Lastena Rafinesque, Ann. Gen. Sci. Phys. Brux., p. 316. 
1853—Leptodea (Raf.) Conrad, Pr. Ac. N. Sci. Phila., p. 262. 
ANIMAL CHARACTERS:—Branchial opening narrow, upcurved, 
papillose, anal smooth, supra-anal short, widely separated from 
anal; gills long, tapering posteriorly, outerand inner gills about 
the same size; inner laminae of inner gills free from visceral mass; 
palpi subfaleate; color of most exposed soft parts orange, rest of 
soft parts tan-colored and soiled white; marsupia, rusty color 
ate 
; 
THE NAIADES OF MISSOURI 450.3 
