Report OF TEE STATS G&OLOGIST. 565 



Volborthia, von Moller. 1873. 



shell convex, very ioequivalve; contour as in Kutorgina. 



Pedicle- valve elevated, incurved at the apex, and with a broad, 



sharply defined cardinal area, divided by a Low convex ridge 



passing from the beat to the cardinal margin. There is no foramen. 



jp% 



Volborthia recurva. After von Moller. 

 Fig. 261.— Profile. Fig. 262 —Cardinal view. 



Brachial valve convex: beak marginal: cardinal area not 

 developed. Muscular impressions not known. Surface covered 

 with concentric lines. Shell substance calcareo-corneous. 



Type, Volborthia recurva, Kutorga (sp.). Lower Silurian. 



Iphidea, Billings. 1872. 

 (Plate 4, Figs. 4, 5.) 



Pedical- valve subconical ; beak apical. Hinge-line short, nearly 

 straight. Cardinal slope flattened, forming an imperfectly 

 defined area, which is divided medially by a convex ridge or 

 pedicle-covering. Foramen circular, apical. 



Brachial valve moderately convex : most elevated at the beak. 

 Internal characters not known. External surface covered with 

 cencentric stria 1 , which are continued over the area of the 

 pedicle-valve. Shell substance corneous. 



Type, Iphidea hell a, Billings. 



Distribution. Cambrian (Newfoundland. Vermont, Arizona, 

 Sweden). 



Acrothele, Linnarsson. 1876. 

 (Plate 3, Figs. 24-27.) 



Shell corneous, composed of several lamina 1 . Pedicle-valve 

 slightly conical, with ex centric umbone, pierced by a minute 

 foramen, in front of which there are two small wart-like pro- 

 tuberances : the area between the umbone and the posterior 

 margin is usually a little flattened. Brachial valve with marginal 

 umbone : in the interior of which there are two oblong, diverg- 

 ing muscular scars, close to the posterior margin, and two small 

 rounded scars near the middle. The muscular scars are separated 

 by a longitudinal ridge. 



117 



