olOKR III 



I'lloTlIKMATA 



313 



Stropheodonta, Hall. Shells very much like Jtiifinesgui-na, but with the 

 cardinal margins finely denticulate and the deltidium flat or not discernible. 

 Silurian and Devonian ; North America and Europe. 



Leptostrophui, Hall and Clarke; DouvUlina, Oehlert and Brachyprion, Shaler,' 



are sub-genera of Stri>}"'niltntfa. Silurian and Devonian. 



Fio. 517. 



.1, Ritfinesquina alternata, Conrad sp. Ordovician ; Cincinnati, Oliio. 

 of ventral valve, showing muscular and vascular impressions. 



BJR. expansa, Sow. sp. Interior 



Pholidostrophia, Hall and Clarke. Smooth or squamose, nacreous small 

 Stropheodontae. Devonian ; North America and Europe. 



Strophonella, fiall (Amphistrophia, Hall and Clarke). Eesupinate Stropheo- 

 dontae. Silurian and De- 

 vonian ; North America and 

 Europe. 



Leptaena, Dalman (Lepta- 

 fionia, M'Coy), (Fig. 518). 

 Shells having the characters 

 of Eafinesquina, but the flatter 

 portions of the valves with 

 corrugations and wrinkles. 

 Where these cease, the shells 

 are more or less abruptly 

 and often rectangularly deflected. 



FIG. 518. 



I^ptaena rhomboidalis, Wahlenb. sp. Silurian ; 

 A, B, Dorsal aspect and profile. C, Interior of dorsal va 



Gottlancl. 



Ive. 



Fio. 519. 



Plectambonites transversal is, Dalm. sp. Silurian ; 

 Gottland. A, Dorsal aspect, 1/1- #> Interior of dorsal 

 valve, 1/1- C, Ventral valve, 3/ x . (A, Adductors; 

 /.', Divaricators). 



Leptella, Hall and Clarke. 

 Lower Ordovician ; North America and England. 



Ordovician to Carboniferous. 



Cadomelldj Munier-Chalmas. Similar 

 to Eafinesquina, but in the ventral valve 

 between the divergent teeth there is a 

 bowl-shaped muscular apophysis for the 

 attachment of accessory diductors. The 

 prominent socket walls of the dorsal 

 valve continued 

 apophyses, which 

 freely. 



Plectamlonites, Pander (Leptaena, 

 Davidson and auct.), (Fig. 519). Ordo- 

 vician and Silurian ; North America 

 and Europe. ^ 

 Primitive Plectambonites. Upper Cambrian and 



into curved crural 

 sometimes terminate 



