384 



MOLLUSCOIDEA 



PHYLUM V 



Intermediate 

 Pledamhonites. 



Subfamily A. Eafinesquininae Scliucliert. 



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

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

 Orthotctinae. 



Eostrophomena Walcott. Primitive, small forms with the cardinal process 

 filling 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. 



Leptella Hall and Clarke. Primitive Plectamhonites. Upper Cambrian 



and Lower Ordovician ; North America 

 and England. 



Plectella Lamansky. 

 between Leptella and 

 Ordovician ; Esthonia. 



(?) LamansJcija Moberg and Seger- 

 berg. Ordovician ; Sweden. 



Plectamhonites Pander {Leptaena 

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

 vician and Silurian ; North America 



PlecUxmhonites transversalis (Dalm.). Silurian ; ■, -p, 



Gotland. .4, Dorsal aspect, Vl- i?, Interior of dorsal aUCi liiUrOpe. 



Divadcators':'' ^^""^'^^ ™^"'' '^'* ^^' ^'^'^""'"''^ = ^'' Leptacua Dalman {Leptagonia M'Coy) 



(Fig. 567). Shells having the characters 

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

 wrinkles. Where these cease, ^ ^ n ' 



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. 566. 



Fig. 567. 



Ltptaena rlwmhoidalif (yfa\i\enh.). Silurian; Gotland. J, B, 

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



Fig. 568. 



i, Ilafiiiesqulnaalteni(tta{Com-aa). Ordovician; Cincinnati, Ohio. Vi- 11, E. exjiansa (Sov/eTby). 



Interior of ventral valve, showing muscular and vascular Impressions. 



