I found 18 loose valves of Pandora trilineata, Threeline or Slender Pandora, within a stretch of some 200 m in Port Aransas. No articulated valves were found. I was surprised at how many were on the beach because this species is uncommon at Ward island, Corpus Christi Bay.

The shells on the Gulf-side were larger than the bay-side shells. The largest valve measured about 22 mm (the valve in the first, square photo of a left valve) in length, while shells from the bay were considerably smaller. Compare with a right valve found at Corpus Christi Bay:
http://www.inaturalist.org/observations/3456348

This species has a very compressed (flattened) shell and it is hard to imagine how the animal lives within such a narrow space. The shell is almost half-moon or crescent shaped in outline. The right valve is smaller and flatter than the left valve, with has a slight concavity.

All shells found on this occasion were pearly. Normally, only the interior is pearly and the exterior is cream-white. These shells have been rolling in the waves and lost the exterior shell layers, but they were still in good condition.

See Encyclopedia of Texas Seashells, p. 335 for more details about the species.