I I 54 CRISTADKNS 



Virgiis misoolensis Simpson, Syn., 1900, p. 852. — Haas, Con. 



Cab., Unio, 1910, p. 133, pi. xiii, fig. 3. 



I have given Schepman's very excellent description almost 

 entire. He states that the nacre is much thickened near the 

 anterior ventral part, forming" a callus connecting with an- 

 other running from the umbones to it. He believes it to be 

 most nearly allied to the Unio bcccarianus of Tapperone-Cane- 

 fri. It dififers from that in having a wider posterior ridge, 

 which is not marked off from the rest of the shell. 



For this species, Haas, (1. c, p. 132), has proposed a new 

 section, Leiovirgns. 



Genus CFIRISTADENS Simpson, n. n. 



Ctenodesma Simpson, Syn., 1900, p. 852. 



Shell subtrapezoid to long elliptical, rather thin, compressed, 

 with a scarcely developed posterior ridge and low beaks, whose 

 sculpture is densely zigzag-radial, becoming finely corrugated 

 or nodulous on the disk and extending over most of the shell ; 

 the disk is concentrically striate or sulcate at its outer edge; 

 pseudocardinals one to two in each valve, very diverse in form, 

 but showing a tendency to break up into denticles ; one lateral 

 and sometimes a faint secondary one in the left valve and two 

 in the right ; dorsal scars few, diversified in form, and various- 

 ly disposed in the shalloAv beak cavities ; anterior muscle scars 

 united; nacre whitish. 



Animal unknown. 



Type, Unio bomcensis Issel. 



In the Synopsis I placed under the generic name Ctenodesma 

 two species of Uniones, which I am unable to refer to any 

 other generic combination. This name having been preoccu- 

 pied by Cook in Myriopoda, J change it to Cristadens. 



The species placed here are elongated, somewhat compressed 

 forms, covered with a peculiar, fine pattern of broken, zigzag 

 sculpture. The pseudocardinals are much lacerated, being in 

 some cases cut into numerous teeth. 



