BRACHIOPODA. 289 



relationship to, or identity with Stropheodonta. In his diagnosis of the genus 

 he describes the sheU. as having a crenulated hinge and flat deltidium. In 

 the pedicle-valve are two small teeth connected with diverging ridges 

 which are strongly elevated in front and enclose a transverse muscular area. 

 Beneath the beak are two crests supported by a short median septum ; posteriorly 

 these all unite to form a single apophysis grooved in 

 the middle and uniting with the inner surface of the 

 deltidium. The brachial valve has a prominent bifid 

 cardinal process, and a pair of small protuberances repre- 

 senting the crural plates ; from the base of the cardinal 

 process extends a median ridge which bifurcates ante- 

 riorly enclosing a cavity for the insertion of the ante- ^'^v.^^^I^^Z^'' 

 rior adductors. 



This division may have some value in bringing into association species hav- 

 ing certain slight variations from the type of structure in Stropheodonta demissa. 



A few convex species in the later Devonian {S. inequistriata, S. arcuata, S. 

 Cayuta, Hall, and S. variabilis, Calvin*), correspond with the characters described 

 by Dr. QIjHLERT, in having the muscular area of the pedicle-valve deepened and 

 its edges strongly elevated. In the brachial valve the anterior myophores 

 have the character of two diverging crests. 



The genus Stropheodonta, though more prolific in species than Rafinesquina, 

 bears much the same relation to Devonian faunas as the latter does to the 

 Silurian. Its earliest members (Brachyprion) appeared while Rafinesquina still 

 existed, but the advent of the typical Stropheodontas was preceded by the 

 decline and extinction of the Rafinesquinas. This fact is true not of the Amer- 

 ican Palgeozoic alone, though it is here best exemplified on account of the far 

 greater abundance of forms belonging to both groups. 



species must belong to the same group of shells. Both species occur associated in the Devonian of the Bas- 

 Boulonnais, in France, and the environs of Voroneje, in Russia. 



* This is a xieculiarly variable shell exteriorly, sometimes with the convexity normal, at others with the 

 convexity reversed nearly as in Stkophonella, and again the valves are at times nearly flat, as in S. per- 

 •plana. 



