Illinois Natural History Survey - University of Illinois

Alasmidonta marginata (Say, 1818)



Alasmidonta marginata, INHS 11754
Alasmidonta marginata, INHS 11754. Blackberry Creek, Kendall County, Illinois. 
Length: 2.8 inches (7.1 cm).

Other common names None.

Key characters Bright yellowish green shell with numerous dark green rays and spots; a sharply angled posterior ridge, poorly developed teeth, and heavy beak sculpture.


Similar species Snuffboxslippershell musseldeertoe.

Description Shell elongate, triangular, inflated, and relatively thin. Anterior end rounded, posterior end sharply angled, ending in a blunt, squared point. Posterior ridge sharply angled and prominent, posterior slope broad, flat, and covered with fine ridges. Ventral margin straight to slightly curved. Umbos large, located near the center of the shell, and elevated above the hinge line. Beak sculpture of three or four heavy, double-looped ridges. Shell smooth and dull. Periostracum yellowish green or bright green with numerous rays and dark green spots present. Posterior slope often lighter than rest of shell. Length to 4 inches (10.2cm).<em>Alasmidonta marginata</em> distribution

Pseudocardinal teeth thin and elongate; one in right, occasionally two in the left. Lateral teeth reduced to a thickened swelling along the hinge line. Beak cavity moderately deep. Nacre bluish white, occasionally with salmon near the beaks.

Habitat Medium-sized streams in gravel or mixed sand and gravel.

Status Widespread but sporadic in distribution. Threatened in Iowa.

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