170 PALEONTOLOGY OF NEW YORK. 



the pedicle-muscles and the modification produced ])y the presence of the 

 pedicle-passage, abstracted. 



In Crania and Craniella, where the sedentary habit appears to be no 

 more than a specific character, there is little variability in the nature of the 

 muscular scars ; in the genus Puolidops, however, the shells of which are 

 invariably' Iree, there is a concentration of the muscular scars at the middle of 

 the valves, the strength of the impressions indicating that considerable power 

 was required to keep the valves closed, with the muscles placed at this dis- 

 a<lvantageous position. 



