ANGULAR RIDGES, HINGE, ETC., IN BIVALVES. 
45 
The Gonhim is the lower posterior angle and the junction of the 
posterior and ventral margins; the Gonial Ridge is the elevated por- 
tion resembling an indistinct and blunt keel, extending from the 
Fig. 109. — Bivalve showing Gonial Ridge extending from the Umbo to the Gonium, where 
the posterior and ventral margin.s join. 
Unio pictorum (L.), right valve, 
Hether.sett Lake, near Norwich, collected by Mr. A. G. Stubbs. 
Gonium to the umbo, and marking off the gonial area ; it is some- 
times very noticeable in Anndontd and Unio, and partly corresponds 
to the Area of many authors. 
The Podium is the lower anterior 
angle, and indicates the point where 
the anterior and ventral margins 
merge ; the Podial Ridge is the 
obsoletely angular line running from 
the umbo to the podium, and cut- 
ting off the podial area from the 
sliell generally. It is strongly 
developed in Dreissensia. 
The Hinge uniting the two valves of the .shell is situate on the 
dorsal margin and is formed by the chitinons liagment and a more 
or less complicated series of denticles or teeth, which closely inter- 
lock with each other and would appear to function as a fulcrum in 
the opening and closing of the valves ; the locomotive bivalves 
generally have the strongest and most powerful hinges, the .sedentary 
and fixed forms usually having comparatively weak liinges and being 
.sometimes (piite edentulous. 
The Hinge Teeth or Denticle.s are more or le.s.s complex, shelly, 
denticular processes or teeth, which with the ligament articulate the 
Fig. 110. — Bivalve showing Podial 
Ridge extending from Umbo to Podium at 
junction of anterior and ventral margin. 
Dreissensia poly7norpha (Pallas), 
Baker's Dock, Stourporr, 
Collected by Mr. J. W. Williams. 
