HO. 1205. SYNOPSIS OF THE NAIADES SIMPSON. 603 



' Obovaria striata RAFINESQUE, Ann. Gen. Sci. Brnx., V, 1820, p. 311. 



* Obovaria torsa RAFINESQUE, Ann. Gen. Sci. Brux., V, 1820, p. 312, pi. LXXXII, figs. 



1,3. 



* Obliquaria retusa RAFINESQUE, Ann. Gen. Sci. Brux., V, 1820, p. 306, pi. LXXXI, 



figs. 19, 20. 



Genus PLAGIOLA (Rafinesque, 1819) Agassiz. 

 (Type, Unio securis Lea.) 



Plagiola RAFINESQUE, J. de Phys. Chim. Hist. Nat., 1819, p. 426. AGASSIZ, Arch, 

 fur Nat., 1852, p. 48, redefined. 



Shell triangular ovate, somewhat inflated, solid, with a distinct and 

 often sharp posterior ridge; surface concentrically sculptured; umboual 

 area somewhat flattened; beaks high, sculptured with delicate, parallel, 

 doubly looped ridges, the anterior loop rounded, the posterior angular; 

 pseudocardinals ragged; laterals club-shaped, straight or slightly 

 curved; cavity of the beaks moderate, often somewhat compressed; 

 nacre silvery; female shell more or less inflated at postbasal region. 



Animal with the outer gills narrow in front, wide behind; inner gills 

 wide in front, posteriorly free or united to abdominal sac; marsupium 

 occupying the posterior part of the outer gills, but not extending quite 

 to the hinder end, consisting of well-marked ovisacs which are rounded 

 below; a distinct sulcus extends the whole length of the kidney-shaped 

 marsupium at the inside and outside at some distance above its base; 

 mantle edge thickened and somewhat doubled, in some cases toothed or 

 fringed below. 



Subgenus PLAGIOLA (Rannesque, 1819) Agassiz. 



Shell solid, surface irregularly concentrically ridged; epidermis 

 smoothish, but here and there wrinkled; painted with larger and 

 smaller scattered rays, which are generally broken into irregular lunate 

 or squarish blotches; hinge heavy and strong; hinge plate wide and flat; 

 female shell smaller than the male, more inflated, and swollen at post- 

 basal region. 



Animal with the branchiae rounded below, inner the larger, wholly or 

 partly free from the abdominal sac; marsupium large, projecting far 

 below the inner gills; mantle very thin, dark on its thickened, fringed 

 double edge; anal opening with very small papillae. 



t PLAGIOLA SECURIS Lea. 



* ? Obliquaria (Plagiola) depressa RAFINESQUE, Ann. Gen. Sci. Phys. Brnx., 1820, 



p. 302, pi. xxxi, figs.5-7. 1 



*Unlo securis LEA, Tr. Am. Phil. Soc., Ill, 1829, p. 437, pi. xi, fig. 17; *Obs., I, 

 1834, p. 437, pi. xi, fig. 17. *DESHAYES, Encyc. Meth., II, 1830, p. 578. 

 SHORT and EATON, Transylvania JL, 1831, p. 77. *DESHAYES, An. sans 



1 Rafinesque's name depressa, for what is probably a male of this species, has prece- 

 dence over Lea's name, but it was used previously by Lamarck for an Australian 

 .Unio. Rafinesque placed this species in his genus Obliquaria and the subgenus 

 Plagiola, but after giving the name Obliquaria depressa he calls it U. depressa in paren- 

 thesis. 



