|Glycymerididae, the bittersweet clams, have round or ovate shells with a hinge area that is wide and arched and teeth that are curved.. They are similar to the ark shells in that they have "taxodont" dentation, meaning they have numerous small teeth that run the length of the hinges. The teeth in the bittersweets are angled upward and outward from the beaks. Bittersweets cannot attach to hard substrates because they have lost the ability to produce byssal threads. Instead they burrow partially in the sandy substrate. They are more active at night. There are 4 species found in Texas waters.