Oyster
(Pelecypoda)
Agerostera (Ostera) mesenterica Morton
A. falcata Morton
Description: The valves of both species of oyster have a distinctive "U" shape. The hinge line is straight and, in the upper valve, projects past the margin of the shell as small flaps.  The lower valve lacks these flaps. The best way to tell the top valve from the bottom is to note the position of the hinge.  Looking at the outside of each valve, the hinge on upper valves will be on the left while the lower valves will have the hinge on the right. The inner margin of the shell is marked with small ruffles or plications.  The outer margin has anywhere from 4 to 15 large plications that end in teeth-like projections.  In A. falcata, these plications extend to the hinge line.  Both valves are similar in size and shape.  The surface of the shell can be marked with concentric growth lines while the inner surface is smooth except for the muscle scar.

Commonality: A. mesenterica occurs in moderate numbers in the oyster beds of the Navesink formation at Big Brook. A. falcata is found as molds and casts in the Mt. Laurel formation and is not quite as common as A. mesenterica.

Similar fossils: A. mesenterica can be confused with A. falcata, but several features can eliminate any confusion.  Though A. falcata does occur at Big Brook, they occur mostly in the Mt. Laurel and then as molds and casts. Further, A. mesenterica differs in that the plications do not extend to the hinge as they do on A. falcata.

Size: Agerostera is small to moderate in size, about 3 cm (about 1.25 in) long.

Notes: Some paleontologists think that Agerostera had the plications in the shell as a modification to rid the soft organs inside of sand and clay that accumulated. They would help prevent the valves from disarticulating while flapping.

A. mesenterica is a fairly good indicator of the upper portion of the Monmouth group (Navesink & Red Bank) in New Jersey being absent only in the Mt. Laurel formation where A. falcata dominates. They may be uncommonly found articulated when found in situ. Some older sources label the genera of this oyster as Ostera. This is an older assignment and can be disregarded.
Upper and lower valves of
Agerostera  mesenterica
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