Snails are rare at this locality. These photos were taken at the large rocks that border the parking area and rim just east of the Bright Angel trailhead.
This is a scaphopod, the faint, elongated cone to the left of the penny. Both this and the snail above are molds—open spaces where the actual shell dissolved away. The scaphopod’s foot and feeding tentacles emerged from the wider end of the cone, much like a snail with a straight shell.