FOSSIL BIVALVE SHELLS. ~ Oke 
simply conical or spiral (except in Patella) is over the hinder 
part of the animal: when the shell is placed on its mouth 
with the apex towards the observer, the parts of the shell 
correspond with the position of the person looking at it. | 
- Lien. 121. ILLUSTRATION OF FossIL BIVALVE SHELLS; nat, 
PETRICOLA PATAGONICcA. D’Orh. 
Interior of right valve, and the same valve with the animal as seen on the 
removal of the left valve. 
a. Labial Palpi. j. Anterior adductor, 
b. Mantle. k. Ligament. 
c. Margin of shell. 1. Umbo. 
d. Branchie. m. Lunule. 
e. Anal siphon. nm. Base, or ventral margin. 
f. Branchial siphon. o. Anterior side. 
g- Foot. p. Posterior side. 
h. Retractor muscle of siphons. q. Pallial line. 
i. Posterior adductor. 
(The length of the shell is estimated from o to p, its breadth from / to n.) 
In bivalves (Lign. 121) the ligament is always on the 
dorsal surface of the animal, and the mouth in front of the 
; apex or umbo of the valves, before the ligament. A bivalve 
placed with the hinge side uppermost and the ligament 
