84 Bryant Walker • 



centric dark lines. Anteriorly from the middle portion a bold rounded rib 

 occurs just below each rest-period. Dorsal area faintly radiately striate. 

 Color, varying shades of chestnut, lighter (nearly straw color) anteriorly, 

 darker posteriorly, rest stages sharply defined by blackish lines. Umbonal 

 region faintly tinged with green. Periostracum thin, slightly glossy, closely 

 adhering. 



"Nacre anteriorly lustrous white and thickened ; posteriorly thin viola- 

 ceous and brilliantly iridescent. Pallial line not sharply defined and with 

 several concentric stride above it, its anterior portion radiately striated. 

 Anterior adductor and retractor scars separated, deep and rough, posterior 

 scars superficial. Dorsal scars concealed by the incurving of the upper por- 

 tion of the valve. 



"Lateral tooth of the right valve thin, very high, wing-like, striated longi- 

 tudinally. Laterals of the left valve similar to that of the right valve, but 

 smaller, the groove between them being very narrow. Pseudocardinals of 

 •the right valve two, thin, opposite, the upper one being the stronger. Pseu- 

 docardinals of the right (left?) valve coalescing, standing in the same 

 straight line, the anterior one high, slightly curved, the posterior low, its 

 summit irregularly crenulated. The sculpture of each umbo consists of 

 four concentric ridges, highest (almost a nodule) at their posterior ends, 

 and numerous concentric stride." (Marshall). 



Type: Cokcria southaUi Marshall, fig. 223. 



Family DRETSSENSIID^. 



Shell mytiliform, equivalve, of prevailing prismatic substance, ligament 

 subinternal; anterior adductor and pedal protectors inserted on a septum 

 in the beak. Byssiferous. 



Genus CONGERIA Partsch, 1835. 



Type : Congeria subglobosa Partsch. 



Typical Congeria is not represented in our fauna. 



Subgenus MYTILOPSIS Conrad, 1857. 



Shell mytiliform, attached by a byssus; hinge with a septum, beneath 

 which on the cardinal side is a triangular cup-shap- 

 ed, thin, white process, which projects obliquely 

 towards the cavity of the valves ; cartilage groove 

 rather deep. Edge of the mantle united and ex- 

 tended posteriorly in two distinct siphons and open 

 Fig. 224 on the base for the extension of the foot. 



Type: Mytilus leitcophccafiis Con., fig. 224. 



