﻿PLAGIOLA 305 



one, and two slightly curved laterals; riqht valve with three 

 pseiidocardinals, the middle one the largest, and two laterals, 

 the lower the smaller; beak cavities moderately deep; muscle 

 scars impressed, the anterior ones ragged ; nacre silvery-white 

 The male and female shells differ widely, the former are much 

 the larger and are considerably compressed ; the female shell 

 is somewhat humped, is more or less inflated, is considerably 

 produced at the posterior base and gaps a little in front and 

 behind. 



Length (male) 90, height 66, diam. 35 mm. 



Length (female) 57, height 43, diam. 30 mm. 



Mississippi drainage south into Arkansas ; west into eastern 

 Iowa and Kansas ; Tombigbee and Alabama river systems. 



Type locality, Ohio. 

 Obliquaria (Plagiola) depressa RaFinrsque, x\nn. Gen. Sci. 



Phys. Brux., 1820, p. 302, pi. xxxi, figs. 5-7. 

 Unio seairis Lea, Tr. Am. Phil. Soc, III, 1829, p. 437, pi. xi, 



fig. 17; Obs., I, 1834, p. 51, pi. XI, fig. 17. — Hanley, Biv. 



Shells, 1843, P- 184, pi. XX, fig. 51.— Chenu, 111. Conch., 



1858, pi. XVI, figs. 4, 4a, 4/?; Man., II, 1859, p. 138, fig. 671. 



— SowERBY. Conch. Icon.. XVI, 1868, pi. lxi, fig. 304. 

 Margarita (Unio) sccnris Lea, Syn., 1836, p. 19; 1838, p. 16. 

 Margaron (Unio) securis Lea, Syn., 1852, p. 24; 1870, p. 37. 

 Plagiola securis Smith, Bull. U. S. Fish Com., 1899, p. 291, 



pi. Lxxx. — Simpson, Syn., 1900, p. 603. 

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

 Plagiola lineolata Agassiz, Arch, fiir Nat.. I. 1852. p. 48. 

 Unio lincolatiis Conrad, New F. W. Shells, 1834, p. 70. — Say. 



Am. Conch, ^continuation), VII, pi. i.xvih (no date). — 



KusTER, Conch. Cab. LTnio. 1861, p. 171. pi. i.m, figs. i. 2. 



In some cases the female shell dift'ers so from that of the 

 male that expert students have believed it to be a different 

 s])ecies. I have an apparently fully adult female shell before 

 me, wliich is 40 millimeters long. 36 in height and 25 in diam- 

 eter. The males appear to be most abundant. Some Coosa 

 River specimens have a rough, rayless epidermis with more or 

 less wrinkled loops. They are destitute of any pattern of 



