hello - I have to model a clamshell and wanted to ask advice from experts on how you might approach - attached a few photos ..
many thanks
rich
a bit of a fiddle to get it right. important is here that you take a lot of care in your first steps creating the overall curves for the shape. the process to redevelop the finished shaped after that is not extreme but it will still help you in the end. whatever you do it will be a bit there and back till you are happy so buckle up 
start with an overall boundary curve without the ridges. or you create it manually and skip the next part.
offset your boundary and one subcurve to get the offset distance. now use divide with something like 50 points. do the same on the offset curve but shift the starting point by the margin you determined before.
now use CurveThroughPt but skip the the few points at the left side here to get into a more even portion otherwise its just ridges all around.
mirror the curve resize it and offset it upwards including rotating it. you might have to play a bit till the endresult suits your need, but turn on history then you can still adjust the shape after lofting.
now extrude both shapes and loft them with matched endings. here i used blendsurface on the small part on the left splitting it to get a more gradual ending. but you experiment how you believe its better to cap it organically. this part is a little tricky though because it also dictates the overal shape a lot.
after this you can use Offsetsrf to make a proper volume, but if you have your small part messed up it may be tricky so you have to experiment.
finally some shrinkwrap to make the surface more homogenous, organic., though that would be a mesh from here.
one last though tip, since the shell shrinks backwards very gradually, you may have to decrease the extrude bottom part of the small part and start bigger as i did, then the loft will be steeper like in your original photo. also you may have to make the ridges deeper so offset the curve more and make more ridges, but i leave that do you.
have fun!
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What have you tried so far? Where are you stuck? Easiest would be to use SubD. That way, it’s also easy to include natural deviations from too perfect symmetry, and you can futz around nicely.
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looks nice maybe you want to hint out a few major steps to get him going.
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Basically very simple not too regular curves, rotated/scaled, a bit rectangular-ish, see photos, then SubD loft, and some futzing like here. Of course, the curves in the vid are far too dense, but that’s one way to do some squishy-squashy forms like these. But as you showed, a shell could also be modelled with NURBS surfaces.
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I derived this 3D model from your photo, then I reduced the face count because it was too heavy (about 30 MB). Now, you could convert it to SubD using quads and manage it much more efficiently.
conchiglia.stl (4.2 MB)
Alternatively, you could create several sections on the mesh and rebuild the surfaces using a loft. There are different approaches you could choose.
Unfortunately, this AI is free. As you can see in the Italian forum, I sent a message to the developers about the issue of not being able to upload multiple photos for the same model, which would allow for a more accurate, fully rounded object. Still, it’s a start.
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thank you all for the kind help - I have many options now moving forward - big thanks
make a point and maybe a circle slightly higher than the point use the edge of your current ending and loft.
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thank you again for taking the time to explain this - much apprciated - rich