EXPLANATION OF PLATE 12. 



Spirodomus insignis, Beecher. 

 Page 162. 



Fig. 1. A transverse section, taken from about 45'"™ from the anterior end of 



a specimen. 

 Fig. 2. A transverse section, posterior to the middle of the specimen fig. 4. 

 Fig. 3. Id. Section of the conjoined valves, at about the posterior quarter of 



their length. 

 Fig. 4. Id. Ventral aspect of a specimen, which is a partial cast of the interior, 



showing the form of the shell, the muscular scars, pallial line and 



crenulated margins. 

 Fig. 5. Id. The left side of the embedded specimen in the rock, showing the 



expanded crenulate margins of tlie valves and the pallial line. 



Waverly group. Warren, Pennsylvania, 



