No. 109.] 



67 



specimens wouU probably show fine concentric lines ; but those hitherto 

 found, have such markings only near the margins of the valves. As is usual 

 in this type o^ Rhynchonella, there is on each side of the plications on the 

 front of the shell a faint longitudinal impressed line. 



In forih and general aspect this species varies greatly, being sometimes 

 longitudinally. ovate or oblong, in others globose to subpentagonal. The 

 extremes of these varieties, without the intermediate forms, would appear 

 to present well-marked specific dificrences ; but a careful study of the 

 series shows such an imperceptible gradation of form as to leave little doubt 

 of their identity. 



Geological position and locality. Pentamerus limestone of the Lower 

 Helderbcrg group, Albany and Schoharie counties. 



EHYNCnONELLA MUTABILIS. 



