CLASS II 



BRACHIOPODA 



381 



JFimanella Walcott. Like Billingsella but more primitive in that the 

 exterior is smooth. Lower Cambrian ; North America. IVynnia Walcott. 

 Middle Cambrian ; India. 



Subfamily C. Eoorthinae Walcott. 



Derived Billing sellidae in which the ddthyria are nearly always loidely open as 

 in Orthids ; deltidia and chilidia sometimes retained throughout life, but more often 

 only in the younger growth stages. Spondylia absent. Cardinal process well 

 developed. Differ from the Orthidae mainly in that the shell structure is dense, 

 granidar, and with irregularly punctate lamellae. 



Eoorthis Walcott. Very much like Plectorthis, but the shell is thinner 

 and its structure not fibrous. Middle Cambrian and LoAver Ordovician, but 

 essentially of Upper Cambrian time ; North America, China, Argentina and 

 north Europe. Subgenus Orusia Walcott, typified by Orthis lenticularis 

 Wahlenberg. Upper Cambrian of north-western Europe and New Brunswick. 

 Subgenus Finkelnburgia Walcott, has thick shells with strongly -marked ventral 

 vascular trunks. Upper Cambrian ; North America. 



Family 2. Orthidae Woodward. 



Progressive, divergent and terminal Orthacea, derived out of the Eoorthinae, 

 nearly always with large open delthyria. Cardinal process well developed. Shell 

 structure fibrous, impunctate or punctate. Ventral muscle area small, obovate or 

 obcordate ; adductors extending to anterior margin of area. Ordovician to Permian. 



Subfamily A. Orthinae Waagen (emend.). 

 Orthidae with the shell impunctate. 



Orthis Dalman (s. str.) (Orthambonites Pander). Typified by 0. callactis 

 Dalman, or 0. tricenaria Conrad. Shells plano-convex ; costae strong, few, 

 generally sharp and but rarely bifurcating. Cardinal process a thin vertical 

 plate. There may be a flat apical deltidium. Plications often with large 

 oblique tubules penetrating the external layers. Ordovician and Silurian ; 

 Europe, North America, etc. 



Plectorthis Hall and Clarke. Valves subequally convex. Ventral cardinal 

 area low. Plications strong, simple or duplicate. Ordovician and Silurian ; 

 North America and Europe. 

 The following are Ordo- 

 vician subgenera : A'ustin- 

 ella, Eridorthis, and Encu- 

 clodema Foerste (Cyclocoelia 

 Foerste, no7i Duj.). 



Platystrophia King (Fig. 

 562). Contour spiriferoid ; 

 hinge-line long with dorsal 

 and ventral cardinal areas 



Platystrophia lynx (Eichw.). 



Fig. 562. 

 Ordovician ; 



Cinciunati, Ohio. 



equally developed. Strong, sharp plications with the exterior surface finely 

 granulose. Ordovician and Silurian ; Europe and America. 



Hebertella Hall and Clarke. Shells with convexity of valves reversed. 



