356 NORTH AMERICAN INDEX FOSSILS. 



striae. Brachial valve with a strong cardinal process from whose 

 base extends a thin longitudinal septum half way down the valve. 

 Muscular area broadly ovate. Devonic. 



A. Width exceeding length *. 



* Median sinus in both valves 421. M. Bella. 



* Median sinus in pedicle valve only ." 424. M. araiata. 



B. Length exceeding width **. 



** Anterior margin truncate I. 



I. Small. B^ak of pedicle valve extended 426. M. barrisi. 



1. Large. Beak of pedicle valve closely incurved 422. Af. Icevis. 



** Anterior margin more or less prolonged into a linguiform extension 2. 



2. Greatest width of shell anterior to middle. Sinus in pedicle valve. 



423. M. princeps. 

 2. Greatest width of shell near middle. No sinus in pedicle valve. 



425. M. nasuta. 



421. M. bella (Hall). Devonic. 

 Valves of nearly equal convexity and both marked by a narrow 



mesial sinus which in meeting give the front an emarginate char- 

 acter. 



Helderbergian of New York, Ohio, New Brunswick. 



422. M. laevis (Vanuxem). (Fig.- 468.) Devonic. 

 Ovate, gibbous. Somewhat resembles M. bella but differs in its 



Fig. 468. Merhlella Icevis (Pal. N. Y., III.). 



greater length proportionally and in the absence of a sinus on the 

 brachial valve. 



