of Burling-ton, loiva, and its Vicinity. 229 



gin, forming the dental sockets, which are open in front. 

 Surface nearly smooth; only a few concentric lines of 

 growth are observed upon the specimens obtained. It 

 is not abundant. 



Geological formation and locality. In the lower portion 

 of bed No. 7 of the preceding sections, Burlington, Iowa. 



ATHYRIS CRASSICARDINALIS (White.) 

 Shell subquadrangular, or subcircular, more or less gib- 

 bous, greatest width usually above the middle, hinge-line 

 broadly curved, equal to about half the width of the shell. 

 Ventral valve convex, beak small, curving upward, and 

 meeting the beak of the dorsal valve. Teeth distant, 

 pointing inward, and slightly upward and backward. 

 Muscular impression rather large, suboval, extending to 

 the beak without interruption. A faint longitudinal ridge 

 is sometimes observed on the inside of the shell, running 

 from beak to front, and a corresponding depression along 

 the outside, which, with a similar depression on the other 

 valve, give a slightly truncated or emarginate appearance 

 to a nan-ow portion of the front margin. Dorsal valve 

 depressed-convex, usually a little flattened at the sides 

 below the hinge-line, much more convex longitudinally 

 than transversely. Beak slightly prominent, meeting the 

 under side of the beak of the opposite valve, and complet- 

 ing the foramen. 



Muscular impressions occupying a long, narrow space, 

 extending more than half the length of the shell. Hinge- 

 plate strong, prominent, and compact, its muscular cavi- 

 ties distinct, more or less deeply notched in front, and per- 

 forated by a very minute foramen ; on each side of it is 

 an oval-shaped cavity of considerable depth for the recep- 

 tion of the teeth of the opposite valve, forming a pivot- 

 hinge so perfect that the valves could not be separated 



