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NEW YORK STATE MUSEUM 



except in rare instances where additions are introduced. There is 

 considerable difference in the coarseness of the radial marking in 

 mature shells, the number being as low as 40 and as high as 80 to 

 90 on each valve. 



The radial lines are crossed by exceedingly fine concentric striae. 

 On the interior the ventral valve shows a deep muscle scar and 

 strong dental plates, the former not being striated by the plications 



Rensselaeria stewarti 

 The four upper figures somewhat enlarged 



of the shell. On the dorsal valve is a defined cardinal area, per- 

 forated hinge plate and narrow elongate muscle area divided by a 

 faint median septum. 



We have spoken elsewhere of the relations of this and similar 

 shells to Trigeria and of the presence of such forms both in the 

 Oriskany and Helderberg faunas. We have identified in the Cumber- 

 land Oriskany, Trigeria gaudryi Oehlert [see Paleontology 

 of New York, v. 8, pt 2, pi. j6, fig. 6, 7] and T. portlandica 

 Billings from Square Lake, Me. is a somewhat similar shell. Both 

 however lack the specific characters of the shell before us. 



Lower Devonic. Dalhousie, N. B. 



