384 



MOLLUSCOIDEA 



PHYLUM V 



Subfamily A. Eafinesquininae Sclmchert. 



Strophomenids that as a rule throughout life have the ventral valve convex and 

 the dorsal cbncave. The relative form of the valves is the reverse of that in the 

 Orthotetinae. 



Eostrophomena Walcott. Primitive, small forms with the cardinal process 

 Alling most of the dorsal delthyrium. Teeth inconspicuous, with the muscular 

 scars almost absent. Surface finely striate with alternating bundles of fine 

 lines between single coarser ones. Basal Ordovician ; Sweden and North 

 America. 



Primitive Pledamlonites. Upper Cambrian 

 and Lower Ordovician ; North America 

 and England. 



Plectella Lamansky. Intermediate 

 between Leptella and Plectambonites. 

 Ordovician ; Esthonia. 



(?) Lamanshja Moberg and Seger- 

 berg. Ordovician ; Sweden. 



Pledamhonites Pander (Leptaena 



Davidson and auct.) (Fig. 566). Ordo- 



^,., . vician and Silurian : North America 



Leptella Hall and Clarke. 



Fig. 566. 



Plectambonites transversalis (Dalm.). ^jinman , , -y-, 



Gotland. ^, Dorsal aspect, i/i- -B, Interior of dorsal and Europe. 



Svar'icfeors'' """'''' ""'""' '^' ^' ''^''"'''" ' ""' Leptaena Dalman (Leptagonia M'Coy) 



(Fig. 567). Shells having the characters 

 of Bafinesquina, but the flatter portions of the valves with corrugations and 

 wrinkles. Where these cease, 

 the Shells are more or less 

 abruptly and often rect- 

 angularly deflected. Ordovi- 

 cian to Lower Carboniferous. 

 Bafinesquina Hall and 

 Clarke (Fig. 568). Shells 

 normally concavo - convex 

 dorso-ventrally. Striae al- 

 ternating in size, and crossed 

 by finer concentric growth 



A 



Fig. 567. 



Leptaena rliomboldalis (Wahlenh.). Silurian ; Gotland. A, B, 

 Dorsal aspect and profile. C, Interior of dorsal valve. 



Fi(i. 568. 



A, Bafinesquina aüernata (Conrad). Ordovician; Cincinnati, Ohio, i/l- B, E. exxwmsa (Sowerhy). 



Interior of ventral valve, showinc; muscular and vascular impressions. 



