Lightning Whelk Egg Case

Busycon sinistrum egg case

These are Lightning Whelk egg casings.  Most likely if you found the lightning whelk egg casing you were along the Gulf of Mexico.  The other options are  either the channel whelk or knobby whelk.  It can be tough to tell which one of the 3 seashell egg casing you have found if they have already hatched.  The Channel whelk 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.  If found in the Atlantic and not pinched it is most likely the Knobby whelk.  Another sure way to tell if looking at each case closely.  The lightning whelk has tiny spikes along the edges of each packet while the knobby whelk is more smooth.  This difference is demonstrated in the photos below

 
Lightning Whelk egg case close up


Lightning whelk egg case


The knobby and Lightning whelk egg casings side by side.  The left one is smoother along the edges making it the knobby whelk while the one on the right has little spikes along the edges making it the Lightning whelk


A collection of lightning whelks.  Notice the opening of this seashell is to the left while most others are on the right


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