382 MOLLUSGOIDEA phylum v 



Exterior finely plicate, crossed by lamellose growth lines. Ventral muscular 

 area short, obcordate. Cardinal process well developed, often crenulate. 

 Ordovician ; America and Europe. 



Orthostrophia Hall. Exterior like Hehertella. Muscular area of both 

 valves short, deeply excavate, with the vascular and ovarian markings 

 conspicuous. Silurian and Lower Devonian ; North America. Subgenus : 



Schizoramma Foerste (Schizonema Foerste, non Agardh.). 



Silurian ; North America. 



Subfamily B. Dalmanelltnae, novum. 

 Orthidae with the shell ahundantly pundate. 

 ^^•'•^^^- Dalmanella Hall and Clarke (Fig. 563). Widely 



Silurian ; Gotiand. i/i- distributed in the ürdovician and öilurian, but persist- 



ing to the end of the Devonian. 

 Thiemella Williams. Upper Devonian ; North America. 



Family 3. RhipidomelÜdae, novum. 



Progressive, divergent and terminal Orihacea with the external charaders of the 

 Orthidae, Shell strudure fibrous, impundate or pundate. Ventral muscular area 

 large, hilohed or elliptical ; addudors relatively small, and more or less completely 

 enclosed anteriorly by the flabellafe didudors. Ordovician to Upper Carbonif erous. 



Subfamily A. Plaesiomiinae, novum. 

 Rhipidomellidae with the shell ii 



Plaesiomys Hall and Clarke. Relative convexity of valves reversed. 

 Surface finely plicate and sometimes tubulose. Cardinal process thickened 

 and crenulate. Ventral muscular scars large and often bilobed. Ordovician ; 

 America and Europe. Subgenus : Dinorthis Hall and Clarke. Surface strongly 

 plicate ; there may be a small deltidium. Ordovician ; North America. 

 Subgenus : Valcourea Raymond. Near Plaesiomys but strophomenoid in external 

 expression. Lower Ordovician ; North America. 



Pionodema Foerste (Bathycoelia Foerste, non Am. Serv.). Biconvex Orthids, 

 finely striate, some of them tubulose. Resemble small Schizophoria but are 

 impunctate. Lower Ordovician ; North America. 



Subfamily B. Khipidomellinae, novum. 

 Rhipidomellidae with the shell abundantly pundate and finely striate. 



Heterorthis Hall and Clarke. Contour strophomenoid with the ventral 

 diductor scars very large. Ordovician ; North America ^^ ^ 



and Europe. 



Rhipidomella QEhlert (Rhipidomys QEhlert, non 

 Wagner). Biconvex, sub-circular shells with short 

 hinge-lines. Striae generally hollow and open on the 

 surface into (1) short tubulär spines. Late Ordovician p^,, r,Q^^ 



into Pennsvlvanian ; widely distributed throughout the BiiohUesbaobus(jjum.). siiu- 



,, -^ ^ ./ o ^j^^. Qotiand. A, Shell, i/i- 



World. B, Interior of dorsal valve, -^/j. 



