This little bubble shell can be dark or light, its eggs white or yellow (the latter being more common).  They can be so prolific in the Spring/Summer that eelgrass beds can be covered in thousands of them, their eggs noticeable on nearly every leaf.  Its shell is quite fragile and it often goes by the common name of Blister Paper Bubble, White Paper Bubble, and most commonly White Bubble Shell. They can often be seen trailing one another as if in a long chain and if you look closely in silty sand along the edge of eelgrass beds, you may be able to see their sand covered bodies. As headshield slugs, they have the ability to use their strong wide heads as a type of shovel to bury themselves in sand.   At times these can be mistaken for the Spotted Aglagid or Aglaja ocelligera, so look closely for distinguishing markers.