Mysterious green mass

An interesting and mysterious green egg mass in the low intertidal zone along the rocky shore of Bodega Head:

 

Here's a closeup view (below):

 

We weren't certain what type of egg mass this was, i.e., what type of animal had produced it.  A worm?  A snail?  So we brought a tiny piece back to the lab for a look under the microscope.  Here's the first view:

 

Tbe egg mass was packed with developing larvae!  Can you tell what they are?

A couple of the larvae (veligers) had hatched and were crawling on the outside of the egg mass:

 

Now you can see that this is a tiny gastropod.  The shell is very tall.  Two black eyespots are visible.

 

The next image shows a nice view from the side:

 

 

We're not sure which species of gastropod this is, but we'll be asking for ideas.  If you know, let us know!