Synopsis Frush-Water Mgli^usca 



49 



ule; beak sculpture consisting of numerous, fine, irregular, broken, some- 

 what concentric corrugations; posterior three-fifths of ihe shell tuberculate; 

 epidermis brown; nacre purple. 



Only the outer gills serving as marsupium. 



Type: Obliquaria (Rotiindaria) tnberculata Raf., fig. 154. Animal, fig. 155. 



Genus PLETHOBASUS Simpson, 1900. 



Shell large, irregularly oval, inflated, solid, somewhat suddenly swollen 

 at the posterior base; posterior ridge low and rounded; heaks rather high, 

 near the anterior end, having a few strong ridges, which are curved up- 



Fig. 156 



wards behind; a row of low, irregular tubercles extends from near the 

 beaks to post-basal part of the valves; epidermis tawny yellow to dark 

 brown ; hinge plate solid, not flattened ; pseudo-cardinals triangular, rough ; 



