﻿3o8 ri.ACiOLA 



U)iio triiiicatiis Sa\, Am. C'oiicli., \ I, 1834; Am. Couch. ( cou- 



tinnalion). \ II, pi. i.wii (no date). — KusTiiR, Couch. Cab.. 



1861, p. 174, pi. lA'. h.^s. 2, 3. 

 Uiiio truiifcitus l\Ai-iN'i:s(H'i'' var. c}r;^aiis r.KTia, Concii. 



Sam., Ill, i8()o, p. 170. 



Shorter, more inflated and more decidedly truncated behind 

 than /'. donaciforniis, towards which it sometimes varies. 



Group of Pla'^iola doiiaciforniis. 



Shell small, elonj.^ated, triangular oval ; posterior ridge 

 generall}- well defined but rarely sharj), having broken green 

 rays, which form chevrons or blotches on a lighter ground ; 

 female shell somewhat inflated in the post-basal region. 



Pl AC. 101. A DONACiFuKMis (Lea). 



Shell rather small, subsolid, irregularly ovate, subinflated. 

 inequilateral, with moderately high and full, though slightly 

 tiattened, beaks, whose sculpture consists of fine, doubly-looped 

 ridges, the hinder loop being quite irregular on the sharp pos- 

 terior ridge ; surface with irregular growth lines, sometimes 

 slightly plicate or corrugated on the posterior slope, generally 

 shining, pale or yellowish-green with a beautiful pattern of 

 darker green ravs. These rays are sometimes entire, but are 

 generallv broken up into arrow-head or zigzag markings; left 

 valve with two compressed i)seudocardinals and two laterals ; 

 right valve with one pseudocardinal and one lateral ; beak 

 cavities shallow ; muscle scars impressed, the posterior ones 

 round ; nacre bluish-white. The female .shell is apparently 

 always smaller than that of the male and has a decided mar- 

 supial swelling, the sharp i)osterior point being a little more 

 elevated than that of the male shell. 



Length (male) 47, height ^2, diam. 20 mm. 



Length (male) iJ, height 23. diam. 16 mm. 



Length (female) 31, height 20. diam. 15 mm. 



Mississippi drainage generally : Alabama River area ; south- 

 west to the Trinity River, Texas ; "Michigan. 



Type locality, Ohio 



