PLKIODOK 1375 



elongated, longitudinally minutely striate, stride occasionally 

 interrupted ; muscular impressions feeble. 



This genus is characterized by an area situated behind the 

 beaks, in the form of a pen, long elliptical in shape, deeply 

 channeled, subangulated. with fine longitudinal strise, some- 

 what interrupted, the widest part is near the beaks, from 

 whence it decreases in size, becoming pointed at the point of 

 contact with the posterior margin." (Rochebrune). 



Type, Arthropteron onassouloui Rochebrune. 



Arthropteron ouassoui,oui Rochebrune. 



"Shell ovoid, rather thick, inequilateral, somewhat gaping 

 below and behind ; pale olivaceous-yellow, irregularly striate,, 

 subfoliaceous sulcate posteriorly ; dorsal margin convex in 

 front, straight behind ; anterior end rather wide, obtuse ; pos- 

 terior rounded, slightly narrow ; beaks contiguous, small, ob- 

 tuse, eroded, situated at 1-3 of the length, area not conspic- 

 uous ; ligament long, somewhat prominent ; nacre pearly, white, 

 here and there rosy-blue. 



Length 69, height .|i, diam. 25 mm." (Rochebrune). 



Type locality. Region of Ouassoulou. French Soudan. 

 Arthropteron ouassouloni Rochkbrune, Bull. Mus. Hist. Nat., 



1904, p. 46T, fig. I. 

 Arthropteron onassonlouensis Germain, Arch. Zool. Exp. et 

 Gen., (5), I, 1909, p. 52, fig. 44. 



Genus PLETODON Conrad, 1854. 



Pleiodon Conrad, Jl. Ac. N. Sci. Phila., VII, 1834, p. 178. 



vShell elliptical, inflated, solid, w^ith a rounded posterior ridge,, 

 and full, smooth beaks ; epidermis smooth, shining, generally 

 rayless ; hinge plate wide, nearly straight, set with strong, ir- 

 regular, taxodont teeth throughout its length ; beak cavities 

 moderate, with a row of dorsal scars running obliquelv down- 

 ward and forward ; anterior and posterior muscle scars well 

 defined. 



Animal with the palpi semilunar, united to the mantle by a 

 straight border, longer than wide; outer gills generally larger 

 than the inner, united their whole length to the mantle ; 



