MONOCONDYL.^iA jogc 



KkY to SPUCIES of lAIONOCONDYL.-IvA. 



Shell more or less roughly, concentrically sculptured. 



Surface with crenate strise. reticulata. 



Surface squamose, rasp-like. jaspidea. 



Shell more or less radially sculptured. 



With strong radial ribs throughout. costnlata. 



With radial depressions in front of the posterior ridge. 



giiarayana. 

 Shell smooth or lightly concentrically striate. 



Nearly orbicular. franciscana. 



Obovate, green, couA-ex. lentiformis. 



Elliptical. 



Solid, subtruncate at dorsal slope. corrientesensis. 



Rather thin, rounded behind. itiermis. 



Subrhomboid or subquadrate. 



Short, almost squarel}- truncate behind. 



niin liana, Paraguay en sis. 

 Long rhomboid, obliquely truncate behind. parchappi. 



Group of MonocondyJaa guarayana. 



Shell obovate. inflated, solid, solid, often slightly produced 

 just behind the center of the base; beaks full, turned inward 

 and forward. 



MoNOCONDYi^A GUARAYANA d'Orbigny. 



Shell oval, inflated, very solid, inequilateral ; beaks full and 

 elevated, somewhat pointed; dorsal outline arched; anterior 

 end rounded ; base line curved in front, slightly emarginate in 

 front of the posterior ridge; posterior ridge imperfectly dou- 

 ble, ending near the base of the shell in a narrow biangula- 

 tion; in front of it are two shallow, wide, radial depressions; 

 dorsal slope obliquely truncate; epidermis brown or yellow- 

 ish-brown ; each valve has a single, rounded, subcompressed 

 tooth, that of the right valve largest ; nacre apparently salmon. 



Length 40, height 29, diam. 24 mm. 



Bolivia. 



