“a 
a ae 
THE NAIADES OF MISSOURI 453 
greatest extent, Walker (rg1oc pp. 75-81) gives us the following 
systematic arrangement of Truncillae on the basis of reproduction: 
1.—Perplexa—Group.—Marsupial expansion occupies the 
whole post-ventral area of shell. 
2.—Triquetra—Group.—Marsupial expansion formed by the 
inflation of the post-umbonal ridge. 
3.—Foliata—Group.—Marsupial expansion anterior to post- 
umbonal ridge and more or less separated from it. 
These groups may in turn be subdivided because of the different 
forms and shifting position of the sexual expansion. The first and 
third groups are represented in this State by two new species, 
discovered by the writer in South Missouri streams, and their 
novelty acknowledged by Mr. Frierson of Frierson, Louisiana. 
Truncilla Curtisii Frierson and Utterback. New Species. 
(‘‘Curtis’ Shell.’’) 
Pl.-VI, Figs. tga-d; Pl..X XVIII, Figs. to9 A—D. 
ANIMAL CHARACTERS. 
NUTRITIVE STRUCTUTES :—Branchial opening densely bordered 
with papillae; anal crenulated; supra-anal high, rather small, 
separated from anal by moderately short mantle connection, 
mantle border here spotted; inner laminae of inner gills entirely 
connected to visceral mass; palpi very small connected only by 
their anterior base which is remote from the anterior attachment 
of pointed outer gill; color of soft parts grayish except the blackish 
mantle edge at siphonal openings. 
REPRODUCTIVE STRUCTURES:— Marsupium very _ broad, 
rounded ventrad, kidney-shaped, ovisacs several, distinct occupy- 
ing posterior half of outer gills and distended into beaded, unpig- 
mented structures along ventral edge when gravid; conglutinates 
broken masses; glochidia unknown as only specimens gravid 
with early embryos have been obtained; inner mantle edge of 
female antero-ventrad to branchial opening drawn in toward the 
interior of shell forming a chamber. 
SHELL CHARACTERS OF FEMALE:—Obvate, lacks flattened 
area of the disc, slightly emarginated just below post dorsal line; 
epidermis cloth-like, brownish-yellow, finely and obscurely radiate 
