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ciilatea , aculeated anteriorly and posteriorly; posterior slope 

 depressed; umbo acutely anD:ulated behind; basal mar:-^in slightly 

 arched; posterior marp-in obliruely truncated; beaks apr)roximate." 

 (Oonrad, 1845) 



" Its sim.lated ribs of trailing blisters give it a 



remarkable similar appearance to Area lactea , which however has 

 real ribs." (Dall, 1886) 



The Bov.'den specimens are small, moderately inflated, subtrape- 

 zoidal in outline, slightly inequilateral; rounded anteriorly, ob- 

 liquely truncated posteriorly, ventral and dorsal margins parallel, 

 the ventral mar-in ve.y slightly contracte'l me-ially; posterior 

 ridge low and broad; ujnbones relatively low, broad, place'! slight- 

 ly anterior to the median horizontal; external sculpture of "blist- 

 ers" more or less connect eci radially and concentrically, forming 

 a delicate cancellated s culpture; beneath the blisters, which are 

 invariably partially or even entirely removed, are faint, slightly 

 elevated radial and concentric lines, as well as irregularly spac- 

 ed concentric lines of growth; cardinal area narrow, excavate-"', the 

 ligament seated on a subiimbonal triangular area; teeth in two ser- 

 ies separated by a narrov subumbonal edentulous space; about fif- 

 teen teeth in the posterior series and eight in the anterior; 

 muscular imToressions well-define 1, bounded by raised rids-es which 

 converge toward the umbones; inner margin of the v Ive usually 

 entire. 



