Family Dreisseniidae 

 Gemis DR2I33EITA Yan Bene den. 



DaLTS^SI'A DOOEIEAM (Kickx) var. j;iI.:AI0J:"3 IS n. var. 



DeGcriotion. - Shell of nediumsize and thickness, out- 

 line var'ing froin lonp-it-ariinally elorr-^te to subfalcate, great- 

 est width slightly oeaiiid c.-.e ::iiddle _ u .ii ;iiell; dreicledl;;? 

 and -unequally convex, the differential Gonve:city caused "bj? the 

 ventral position of the iironinent, hirh, hi^t rounded, umhonal 

 ridge, its axis beinr: ;iL"uuai:e-' elo;:e to ^he veiibral iiiarp;in 

 throiighout; umbo acute, terminal, hent forward; dorsol margin 

 convex, suhanprular to oroadl^v curved throixrrhout ; ventral mar- 

 giii concave throughout r flexecl out\7ard before ;_'oiiiing the 

 abruptly rounded posterior r.arrin; external surface sculntured 

 •"ith irregularly dispose", oGcasi~nally eira?" ', increment- 



al lines, usually most conspicuous ne-^r the r.:ar;-lns, and a few, 

 ohscure, irregular, concentric nrndulat ions ; lunbonal septum 

 hi.p'h and v.l de for the "i-^e --^ tte chell; left vplve "1th a rel- 

 atively large, shallow, veutrally situated 3oa.-:et, co-xei^ .'end- 

 ing tooth in the ri^ht valve ' looed or obsolete; lig- 

 srnent si p-roove promitient, occunyir^- more than a third of the 

 proxinal portion of the dorsal margin. 



Dimensions ,- lHar.. lat, ( dor so-ventral) 9.5 mm.; alt., 

 ( antero-postero) 18 mm.; nemidia... , . ..... 



