Carditamera floridana Conrad, 1838
The Broad-Ribbed Cardita might be mistaken for an Ark Shell (Family Arcidae), but actually Carditamera floridana is a member of Family Carditidae. The adult bivalve is approximately 1½ inches long. The shell is sturdy, bluntly oval, and has about 20 robust radiating ribs. The ribs are wide, beaded or scaly, and have purple or reddish-brown spots scattered over them in somewhat concentric bands. The background color of the shell is white, grayish, or slightly yellowish. Old Carditas may have lost their markings. Fresh specimens sport a gray periostracum. The inside of this type of clam shell is porcelain-white.
The beak, or umbo, of the shell is large, and is situated about a fourth of the way from the rounded front end of the shell. The right valve of the clam has a large central tooth, and the left valve has a smaller central tooth. The lunule, the area in front of the umbo, is depressed, and shaped like a plump valentine. There is a narrow, external ligament which connects the two valves. The edge of the shell is quite scalloped and ridged.
This shell is sometimes known as the Florida Cardita or the Bird Shell. (I suppose it is called the “Bird Shell” because the still-connected valves may be found spread open, giving the appearance [somewhat] of the spread wings of a bird.) — This nickname for Carditamera floridae has, for the most part, fallen into disuse.
Its range is from Florida to Texas and Mexico, in sand or mud 3-25 feet deep. It attaches itself to the substratum by means of its byssus (threadlike filaments).