y. 



190 THE NAIADEvS OF MISvSOURI 



greatest extent, Walker (1910c pp. 75-81) gives us the following 

 systematic arrangement of Triincillae on the basis of reproduction: 



I. — 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. i4a-d; PL XXVIII, Figs, ioq 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, 

 rnantle 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 gra3dsh 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 



