Dr. Gray's definition of the respective parts of univalve and bivalve shells is at once clear, concise, and natural, being conformable to the structure of the body of the enclosed mollusk.
The front of the shell is the part which covers the head of the animal; the back of the shell is the part which covers the tail; the left and right sides correspond with the same parts of the mollusk.
In univalves, the apex of the shelly cone whether it be 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.
FOSSIL BIVALVE SHELLS.
Lign. 121. Illustration of Fossil Bivalve Shells; nat.
Petricola Patagonica. D'Orb.
Interior of right valve, and the same valve with the animal as seen on the removal of the left valve.
| a. Labial Palpi. b. Mantle. c. Margin of shell. d. Branchiæ. e. Anal siphon. f. Branchial siphon. g. Foot. h. Retractor muscle of siphons. | i. Posterior adductor. j. Anterior adductor. k. Ligament. l. Umbo. m. Lunule. n. Base, or ventral margin. o. Anterior side. p. Posterior side. q. Pallial line. |
(The length of the shell is estimated from o to p, its breadth from l 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 towards the observer is in the same relative position as the person looking at it; viz. the head in front, and the right and left valves in their natural relations. The length of the shell is therefore from the front to the back of the animal: the width or transverse diameter is from the umbo to the margin. Much confusion has arisen from many conchologists having described the length and width of a shell diametrically opposite to the proper position of its inhabitant.