ARciDOPSrs 1 191 



posterior ridge prominent, narrowly rounded, curved down- 

 ward in the middle, ending in a produced point near the base 

 of the shell ; posterior end sharply incurved between the wing 

 and the base ; base rounded ; anterior end rounded and cut 

 away below ; surface strongly, concentrically striate, highly 

 corrugated at the umbones ; epidermis pale green, obscurely 

 radiate behind; nacre beautifully pearly, iridescent, pale rose 

 color; pseudocardinals elongated, corrugated, single in the 

 left valve, double in the right ; laterals single in the right valve, 

 double in the left. 



Length 53, height 30, diam. 9.5 mm. 



Lake Mweru, British Central Africa. 

 Unio (Metaptera) johnstoni Smith, Pr. Zool. Soc. Lond., 



1893, p. 640, pi. Lix, figs. 18-20. 

 Pseiidaznciila johnstoni Simpson, Syn., 1900, p. 861. 



This remarkable species bears a striking resemblance to a 

 Frisodon and is possibly closely related to that group. Smith 

 states that the region above -the umbones is lightly corrugated, 

 but does not say whether this sculpture is zigzag-radial or 

 otherwise. He states that the shell gapes slightly at both ends. 



Genus ARCIDOPSIS Simpson, 1900. 



Arcidopsis Simpson, Syn., 1900, p. 861. 



Shell inequilateral, elongated, with dorsal and ventral lines 

 nearly straight and parallel ; anterior end strongly and obliquely 

 truncated from the beaks to the anterior base, and ending in a 

 rather sharp but rounded point below ; posterior end obHquely 

 truncated above, rounded below ; posterior ridge full, rounded ; 

 the shell in front and below rather compressed ; surface con- 

 centrically and radially ridged ; pseudocardinals strong, two in 

 the right valve, one in the left, and supported by a strong, 

 cardinal rib; muscle scars well impressed; pallial line distinct. 



Animal unknown. 



Type, Unio footei Theobald. 



Arcidopsis footei (Theobald). 



Shell trapezoid, elongated, compressed, inequilateral ; beaks 

 prominent ; dorsal line straight, posterior end obliquely truncate 

 above, rather widely rounded below ; base line straight ; anterior 



