an oyster shell, a shell that will contain a pearl (see Molina)
Alonso de Molina:
eptatapalcatl. concha de perla. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 29r. col. 1. Thanks to Joe Campbell for providing the transcription.