382 



MOLLUSCOIDEA 



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, noii Agardh.). 



Silurian ; North America. 



Subfamily B. Dalmanellinae, novum. 

 Orthidae with the shell oMmdantly punctate. 



Dalmanella Hall and Clarke (Fig. 563). Widely 

 distributed in the Ordovician and Silurian, but persist- 

 ing to the end of the Devonian. 

 Thiemella Williams. Upper Devonian ; North America. 



Fifi. 563. 



DuhiMnella elegantula (Dalin.). 

 Silurian ; Gotland, i/i- 



Family 3. Rhipidoraellidae, novum. 



Progressive, divergent and terminal Orthacea with the external characters of the 

 Orthidae. Shell structure fibrous, impunctate or punctate. Ventral muscular area 

 large, bilobed or elliptical; adductors relatively small, and more or less comptletely 

 enclosed anteriorly by the fl,abellate diductors. Ordovician to Upper Carboniferous. 



Subfamily A. Plaesiomiinae, novum. 

 Ilhipidomellidae with the shell impunctate. 



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 : Dvnorthis 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. Rhipidomellinae, novum. 



Pihipidomellidae with the shell abundantly punctate and finely striate. 



Heterorthis Hall and Clarke. Contour strophomenoid with the 

 diductor scars very large. Ordovician ; North America -^ 

 and Europe. 



Ehipidomella ffihlert {PMpidomys tEhlert, non 

 Wagner). Biconvex, sub -circular shells with short 

 hinge-lines. Striae generally hollow and open on the 

 surface into (?) short tubular spines. Late Ordovician pj,. ^g^ 



into Pennsvlvanian ; widely distributed throughout the y./M^ite /,;?,.//».« (l 



world. A', Interior of dorsal 



ventral 



inn.). Silu- 

 Sliell, i/i- 

 \-alve, -/j. 



