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NEW YORK STATE MUSEUM 



fifth the length of the valve though without thickening. There is 

 no impressed muscular scar and no thickening of the shells in the 

 umbonal region. In the dorsal valve, though there is a median 

 septum extending about one third the length of the shell, there is 

 no thickening at the beak and the hinge plate is so slender that we 

 have been unable to make it out. All these details are in notable 

 contrast to R. atlantica, R. stewarti and the Coblentzian 

 species referred to. 



They are in closer correspondence to the Helderberg species 

 R. aequiradiata Hall and indicate, irrespective of their con- 

 siderable size, an entirely primitive condition of development. The 

 markings of the surface consist of simple rounded or slightly flat- 

 tened plications seldom with concentric growth lines or other 

 interruptions. There are about 50 of these simple plications on 

 each valve, the number varying very little with size and age. 



Lower Devonic. Misery stream, first dam in town of Sand- 

 wich ; Brassua lake, opposite Moose river, Me. 



Rensselaeria diania nov. 



This species retains the contour and simple structure of 

 R. c a 1 1 i d a but difl^ers wholly in its exterior which carries 20 to 30 



Rensselaeria diania 



very coarse and broad, sometimes quite sharply keeled plications 

 which meet in sharp interlocking angles at the edge. Its similarity 



