Knobby Whelk Egg Case
Busycon carica egg case
These are Knobby Whelk egg casings. They look very similar to the lightning whelk and channel whelk egg casing. It can be tough to tell which one of the 3 seashell egg casings you have found but there is a difference. First to eliminate the Channel whelk, it can be told from the other 2 because it's edges of each disk are pinched in while the other 2 are more like a coins edge. Another tip is if found in the Gulf of Mexico it is most likely the Lightning whelk. If found in the Atlantic and not pinched it is most likely the Knobby whelk. To be sure though, if you one with a coin type edge instead of pinched, look closely at the edge of one of the packets. If it has little spikes poking out it is the lightning whelk and if it is more of a smooth edge it is the knobby. Confusing in type but the images below really points this difference.
Knobby Whelk egg case close up
The 2 egg casings side by side. The one on the left without the little spikes is the knobby whelk while the one on the right has the tiny spikes making it the lightning whelk
The 3 types of Whelk seashells with this type egg casing that most often wash up.
Lightning, Knobby, and Channel whelks
Baby mini Whelks found in the shell egg case