4G 
THE HINGE TEETH IN BIVALVES. 
sliells. Ill young sliells tliese processes or teetli are sharp and well 
defined, but in aged ones they often become thickened and their 
pecnliar characters even obliterated by the continned deposition of 
shelly matter. These denticles are termed 
Cardinal, or IIinc.e-tketii when placed 
immediately beneath or between the 
Umbones, and Antero-lateral or 
PosTERO-LATERAL according as they are 
situate on the anterior or posterior side 
of the Umbones ; these lateral teeth are 
generally distinctly lamellar in character, 
and sometimes very distinctly and 
strongly developed ; these projecting 
shelly processes are generally arranged 
.so as to fit between, and interlock with, 
corresponding cavities in the opposite 
valve. Occasionally the lateral teeth or 
denticles are developed and not cardinal 
ones, as in Unio, and shells with this type 
of hinge-teeth have been called Priono- 
desmacea by Dali, and Heterodonta by 
Aeumayr; more freipiently, however, in Pelecypods generally, the 
cardinal teeth alone are pre.sent. A formula has been devised for regis- 
tering the different peculiarities of the hinge-denticles, the formula for 
the right valve being jdaced above, and that indicating the dentition 
Left Valine. 
I 
Eio. 112 — Hinge-Teetli of i’nio iuntidus Phil. 
(lrol»y J^ool, Leice-ilersliire, collected l>y Mr. H. K. Quilter. 
nj. antero-lateral lamcli.x; /./, posteru-lateral lamellai. 
of the left valve beneath it, the cardinal denticles being first given, 
followed by the laterals, thus Unit) ttmidns could he recorded as 
Cardinal teeth ; Antero-lateral h ; Postero-lateral ^ ; which formula 
I 
Fio. Ill, — Diagram ofa transverse 
section of the sheil of a Bivalve (after 
Lankester), showing how the hinge 
teeth act as a fulcrum in the opetting 
and closing of the valves. 
a. adductor muscles; /. ligament; 
r.7f. and right and left valves 
of the shell ; A. hinge or fulcrum ; 
f.f. the short arms of the lever ; 
ef.d. the long arms of the lever. 
