358 



NORTH AMERICAN INDEX FOSSILS. 



423. M. princeps Hall. (Fig. 469.) Devonic. 



Large, ovate, ventricose. Pedicle valve depressed anteriorly 

 into a broad mesial sinus, terminating in old specimens in a lin- 

 guiform extension. Brachial valve greatly elevated in the middle, 

 producing a strong mesial elevation anteriorly. Distinguished 

 from M. nasuta in its greater width being anterior to the middle 

 and in the presence of a sinus in the pedicle valve. 



Helderbergian of New York, New Brunswick. 



Fig. 471. Meristelm ,ir<mita, internal mold (Pal. N. V., III.). 



424. M. arcuata Hall. (Figs. 470, 471.) Devonic. 



Broadly ovate to transversely oval. Pedicle valve bearing an- 

 teriorly a shallow depression. Brachial valve gibbous along the 

 middle. Surface smooth except for a few faint radiating and con- 

 centric lines. 



Helderbergian of New York and New Brunswick. 



Fig. 472. Meristella nasuta, Schoharie form (Pal. N. Y., IV.), 



