368 MOLLUSCA SUB-KINGDOM VI 
byssus passing through a narrow gape between the valves which are otherwise closed. Ordo- 
vician to Devonian. 
The typical Ambonychia, according to Ulrich, is edentulous ; the forms ordinarily passing 
under that name being now referred to Bysson- 
ychia. In this group the byssus does not pass 
through a notch in one of the valves. 
The Ainbonychiidae include the typical mem- 
bers of Ambonychia, Byssopteria, and Amphi- 
coelia, Hall; Opisthoptera (Megaptera), Meek ; 
Anomalodonta, Miller; Byssonychia and Allo- 
nychia, Ulrich ; and their allies. 
Byssonychia, Ulrich (Fig. 619). Hinge 
with several small cardinal and two or three 
slender lateral teeth ; area striated ; otherwise 
as in Ambonychia. 
: Paleocardia, Hall. Silurian. Mytilarca, 
Fic. 619. Plethomytilus, Hall. Devonian. 
A, Byssonychia, sp. Cincinnati Group; Cinecin- cop] ota 11 1S APS Pye 5 
nati, Ohio. Interior of right valve. 1/) (after Gosseletia, Barrois (Cyrtodontopsis, Frech). 
Miller). B, B. radiata (Hall), Ulrich. Same locality. Thicker shelled, with heavier and more 
numerous teeth. Devonian. 
Clionychia, Ulrich, Edentulous, concentrically sculptured. Silurian. 

Family 18. Pinnidae. Meek. 
Shell Mytiliform, not alate, dimyarian, the anterior adductor smaller; equivalve, 







Fia. 620. 
; ee . Fic. 621. 
Pinna pyramidatlis, Miinst. : 4 ‘ : 
Quader Sandstein; Schan- Pinnigena Seebachi, Bbhm. Upper Jura; Kelheim, Bavaria. External 
dau. 1/3. and internal views, 1/s natural size. 
truncate, and wholly open behind ; edentulous; area linear; ligament parivincular, in- 
