Mitch, in the small bit of practice I have had with this design, I have always scalloped the ends of the shell when hollowing them out. Your carving looks great, but just different from mine. Not having had a chance to hold an original with this design, I am just guessing from pictures I have studied. I don't know what is correct, which is why I am glad you showed and example of yours.
This is also a hard area for me to carve, because the grain can be a bit difficult to deal with in this area, and chip-out is not uncommon here. Extremely sharp chisels are required in this area, which you obviously have by the look of your cuts. Great looking shell design.
Matt