﻿PLAGIOLA -III 



Unio cognatiis Lea, Pr. Ac. N. Sci. Phila., IV, i860, p. jo6; 



Jl. Ac. N. Sci. Phila., IV, i860, p. 368, pi. lxv, fig. 193; 



Obs., Yin, i860, p. 50, i,xv, fig. 193. — SowERBY, Conch. 



Icon., XVI, 1866, pi. xxxix, fig. 214. 

 Margaron (Unio) cognatiis Le:a, Syn., 1870, p. 43. 

 Plagiola cognata Simpson, Syn., 1900, p. 606. 



I have only seen two left valves of this species, one the 

 type, and a young one. Roth are quite solid, the younger 

 being perfect enough to show something of the beak sculpture. 

 The older one is probably from a female, as it is rather full 

 in the post-basal region. I should say that the species is closely 

 related to P. macrodon and donaciforniis, but it is a heavier 

 shell than either and more inflated than the former. The 

 younger valve shows a decided, sharp posterior ridge, in front 

 of which, especially near the beak, the surface is decidedly 

 flattened or almost excavated as in Plagiola elegans. 



Subgenus Artonaias von Martens, 1900. 



Artonaias von Martens, I>iol. Cent. Am., Mo'lL, 1900, p. 480. 

 Ptychodenna Simpson, Syn., 1900, p. 606. 



Shell triangularly ovate or rounded, solid, bluntly angled 

 before, more sharply angular behind, with a tolerably well- 

 marked posterior ridge ; surface strongly and irregularly con- 

 centrically sulcate • beaks small but rather prominent, sculp- 

 tured with fine, irregular, broken ridges, which are somewhat 

 doubly looped, the front loop being larger and more rounded ; 

 epidermis olive to tawny, wrinkled, sometimes having faint 

 rays ; pseudocardinals compressed, ragged ; hinge plate nar- 

 row ; laterals short, slightly curved, obliquely striated ; muscle 

 scars rather shallow, the posterior round ; female shell pro- 

 duced in the post-basal region. 



Animal having the branchiae small, inner united the whole 

 length to the abdominal sac, wider than the outer in front, 

 narrower behind ; marsupium occupying the posterior part of 

 the outer gills in from eleven to twenty distinct ovisacs, which 

 are rounded and dark below, the whole having a distinct, par- 

 allel sulcus inside and out near its base ; mantle thin, papery. 



