THE HINGE TEETH IN BIVALVES. 
47 
indicates that the cardinal denticles are deficient in both valves, and 
that the right valve contains one antero-lateral and one postero- 
lateral lamella, the left valve containing two on each side, ai-ranged 
so as to interlock with the lamellar ridges in the other valve ; the 
anterior laminm are always more or less cremate at their free margins 
and the more prominent and central denticle of the left valve is often 
distinctly bifurcate, as shown in the illustrative figure. Dr. Jeffreys 
was in error in his account of the denticulation of the hinge of the 
Uiiiones, ascribing the bidentate teeth to the right valve, which in 
normal shells has only unidentate lamellse. 
SijJiwrium rivicola may be considered to typify the shells possessing 
cardinal and lateral hinge teeth, both of are somewhat variable, 
in character. In well developed specimens the cardinal teeth 
are often distinctly double in both 
valves, the posterior cardinal tooth in 
the right valve, and the anterior 
cardinal tooth in the left valve, being 
larger than the other, often with a 
somewhat angular excavation and ap- 
pearing as though formed by two 
converging denticles, their ai)ices 
pointing to the umbo. In the un- 
usually developed specimen from Leicestershire (f. 113), the cardinal 
denticles are more distinctly developed than is usual, and have a 
great resemblance to three more or less sejiarate and converging 
shelly processes ; the anterior lamellae of the right valve are two in 
number, as in the normal shell, but in the left valve, a slightly 
developed additional lamella is perceptible nearer to the umbo and 
the outer .shell-margin. The posterior lamelhe of the right valve 
have also developed an additional ridge also close to the outer margin 
and near to the umbo, affording a groove for the interlocking of the 
extra lamella appearing in the left valve ; this .specimen would 
appropriately be indicated by the formula. Cardinal | ; Antero- 
lateral f ; Postero-lateral |. In ordinary specimens the teeth may 
he formulated as Cardinal#; Antero-lateral f; Postero-lateral 
indicating that there are two cardinal teeth in each valve, and that 
the right valve contains two antero-lateral and two postero-lateral 
teeth, the left valve containing only one at each side interlocking 
Fig. 113 — Hinge-Teeth of Sphccrium 
rivicola (Leach) X 4. 
Canal, Blaby, Leicestershire, 
Collected by Mr. H. E. Quilter. 
c. cardinal denticles; aJ. antero-lateral 
lamellcc; p.l. postero-lateral laniellje. 
