452 THE AMERICAN MIDLAND NATURALIST 
Genus Truncilla Rafinesque. 
(Type, Truncilla triqueter Rafinesque). 
1819—Truncilla Rafinesque, 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 most recent classification of the Unionidae is based 
primarily upon the modification of the marsupium and that the 
genus 7runcilla has carried out this sexual differention to the 
