150 THE NAlADES OF MISSOURI 



genus is in its peculiar shell and glochidial characters. As to its 

 animal characters it is essentially that of Obovaria and Amyg- 

 dalonaias. Because of its spatulate and higher-than-long glocidia 

 there is "a transition," as Dr. Ortmann aptly puts it, "toward 

 the glochidia of the Proptera-type," which has essentially the same 

 hinge line and rounded ventral margin, but with different post- 

 terior and anterior ends. 



Plagiola securis (Lea). 



(Butterfly.") . 

 PL II, Figs. 3a— 6b. 



1829 — Unio securis Lea, Tr. Am. Phil. Soc, III, p. 437, pi. XI, fig. 17. 

 1834 — Unio lineolata Say,' Am. Conch., VI. 



1900b — Plagiola securis (Lea) Simpson, Proc. U. vS. Nat. Mus., XXII, 

 p. 603. 



ANIMAL CHARACTERS. 



Nutritive Characters: — Branchial opening rather small 

 with numerous papillae; anal crenulated; supra-anal very large 

 closely, but definitely connected by mantle edges to anal; anus 

 tentacled; inner laminae of inner gills free about three-fourths 

 of their length from the visceral mass; palpi large, connected 

 one-third of their length antero-dorsad; color of soft parts tan, 

 or cream color with gills brownish and papillae yellowish. 



Reproductive Structures: — Marsupium composed of about 

 twenty-five ovisacs placed in the posterior half of outer gill ; mantle 

 border antero-ventrad to branchial opening with about fifteen 

 very low, irregularly placed papillae; marsupium when charged 

 with ripe glochidia color of brown sugar, somewhat kidney shaped; 

 conglutinates not very solid; glochidia subelliptic or spatulate 

 hinge line very short, ventral margin round, spineless, much higher 

 than long (0.230 x .330mm). 



shell characters. 

 External Structures : — Shell sub-triangular, not very much 

 inflated, compressed in umbonal region, truncated narrowly post- 

 dorsad; disk more or less smooth; beaks somewhat pointed 

 and directed anteriorly, sculptured with a few fine concentric 



' This name, as employed by Say, was really preoccupied by Rafinesque 

 in his Monograph of 1820 and hence through priority this species should 

 bear the name Plagiola lineolata (Raf.) in spite of the fact that Simpson 

 considered this as one of the many "indeterminates " of Rafinesque. 



