CLASS I 



PELECYPODA 



445 



Family 17. Ambonychiidae Miller. 



Shell mytiliform, with no anterior wing, the anterior adductor obsolete; equivalve, 

 very inequilateral ; dentition obsolete or schizo- 

 dont ; licjament external,^ multivincular (?) ; 

 byssus passing through a narrow gape hetween 

 the valves which are otherwise closed. Ordo- 

 vician to Devonian. 



The typical Amhonychia, according to Ulrich, 

 is edentulous ; the fornis ordinarily i)assing under 

 that name being now referred to Byssonychia, In 

 this group the byssus does not pass through a 

 notch in one of the valves. 



The Ambonychiidae include the typical meni- 

 bers of Amhonychia, JByssopteria and Amphicoelia 

 Hall ; Opisthoptera {Megaptera) Meek ; Anomalo- 



dontamm^v, Byssonychia and AUonychia UMch, a, Byssonychia sp. Cincinnati Group; Cin- 

 and their allies. cinnati, Ohio. Interior of right valve. V, (after 



Miller). B, B. radiata (Hall). Same locality. 



Byssonychia Ulrich {big. 668). Hinge 

 with several small cardinal and two or three slender lateral teeth ; area striated ; 

 byssal opening present in tlie upper half of the anterior face ; otherwise as in 

 Ämbonychia. 



Silurian. Mytilarca and Plethomytilus Hall. Devonian. 

 {Cyrtodontopsis Frech.). Thicker shelled, with heavier and 

 Devonian. 

 Edentulous, concentrically sculptured. Ordovician. 



Paleocardia Hall. 

 Gosseletia Barrois 

 more numerous teeth. 

 Glionychia Ulrich. 



Pinna pyramidalis Münst. 

 Quader Sandstein ; Schan- 

 dau, near Dresden. 1/3. 



Fig. 670. 



Pinnigena scehachi Böhm. Upper Jura ; Kelheim, Bavaria. 

 External and internal views, 1/3 natural size. 



