Comparing two sea snails

I have a long list of things I've been meaning to post, so I'm going to check one of them off my list tonight.

Remember the Violet Sea Snail (Janthina janthina) that I showed on 16 March 2016 (see post here)?  I thought it would be useful to post better pictures of its shell, especially in comparison to the related Purple Sea Snails (Janthina umbilicata) shown in January and February 2016.

Here's the first photo of the Violet Sea Snail (Janthina janthina), showing the view with the aperture (opening) of the shell facing up:

 

And here's a view from the opposite side:

 

Note how pale Janthina janthina is, especially on the topmost whorls.  Also examine the shape of the striations (lines) on the shell.  They bend gently (in a U-shape) along the center of the largest whorl.  We'll compare these lines in both species soon, but first here's another view of these two features (the paleness and the rounded lines):

 

To compare Janthina janthina with Janthina umbilicata, I'll show them in the same position.

 

So here's Janthina janthina:

And here's Janthina umbilicata:

Now you can compare several different features:

(1) the height of the spires.  Note that the spire (topmost whorls) is much lower in Janthina janthina — it appears flattened in comparison with the taller spire in Janthina umbilicata. 

(2) the shape of the striations.  Look for the gently rounded, U-shaped striations in Janthina janthina.  Compare that shape to the strong angled V-shaped striations in Janthina umbilicata.  (Follow the striations from the top of the whorl down towards the bottom.)

(3) a keel in the center of the largest whorl.  Janthina janthina lacks a keel in the center of the largest whorl — you only see the striations. While Janthina umbilicata has a noticeable keel — in the photo above, it looks like a zipper running through the center of the whorl (located at the bottom of the "V").

Janthina janthina appears to be quite rare in northern California, but now there's a pretty good record of this shell and you'll know what to look for to separate these two species if you are lucky enough to find a sea snail in the future!