104 THE NAIADES OF MISSOURI 



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. conjragosus 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. Conjragosus 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. vSci. 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 subfalcate; color of most exposed soft parts orange, rest of 

 soft parts tan-colored and soiled white; marsupia, rusty color 



