No. 109.] 67 



specimens woull 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 of R/n/nchojiella, thcre is on each side of the plications on the 

 front of the shell a faint longitudinal impressed line. 



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

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

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

 to present well-marked specific differences ; 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 

 Helderberg group, Albany and Schoharie counties. 



RhTNCHONELLA MUTABILIS. 



