TH^ NAIADES OF MISSOURI 189 



Genus Truncilla Rafinesque. 



(Type, Truncilla triqueter Rafinesque). 

 1819 — Truncilla i^afinesque, Jl. de Phys. Chim. et Hist. Nat. pp. 427. 



Animal Characters: — Branchial opening densely set with 

 papillae; anal crenulated; supra-anal well separated from the anal 

 by a definite mantle connection; inner laminae of inner gills 

 entirely connected with the visceral mass; outer gills greatly 

 tapering anteriorly; palpi very small, post-dorsal margins entirely 

 free; color of soft parts grayish, posterior margins of mantle black. 

 Marsupia kidney-shaped, distended transversely as well as ventrad 

 when gravid; formed by many ovisacs that occupy the posterior 

 section of outer gill and extend down from the ventral edge of the 

 original sterile gill in blunt, beaded unpigmented structures in 

 state of gravidity; conglutinates not solid; glochidium medium 

 in size, semicircular, hinge line very long, length about the same 

 as height; post-ventral edge of marsupium doubled, the inner 

 edge remote from outer forming a peculiar compartment as the 

 highest specialization for the respiration of the embryos. 



Shell Characters: — Female shell very distinct from that 

 of male, with a strong inflation or projection (marsupial expansion) 

 in the post-ventral region which so changes its position and form 

 that it assumes very many strange shapes; male shell not so 

 modified post-ventrad, however, nodulous expansions do appear 

 in some species; shell of either sex small, usually narrowly rounded 

 in front much thicker anteriorly; beaks rather full, comparatively 

 large, sculptured by double-looped ridges, usually obscure; epi- 

 dermis yellowish or brownish, rayed with numerous fine green 

 lines; cardinals and laterals usually single (or faintly double) in 

 right valve, double in left; branchial cavity deep, basin-like; 

 nacre white or bluish. 



Miscellaneous Remarks: — This remarkable genus is the 

 most modern due to the best possible adaptation to reproduction 

 both as to the morphology -of shell and soft parts. This correlation 

 of the physiology to the morphology is best seen in the greatest 

 specialization of the reproductive structures of the animal. Recog- 

 nizing that the tnost recent classification of the Unionidae is based 

 primarily upon the modification of the marsupium and that Ihc 

 genus Truncilla has carried out this sexual difi"ercntion to the 



