Need help with a seashell model

Hey! So I'm an Intern at an engineering firm and my boss gave me a small project on solid works. On a sea shell, you can tell how old it is by these certain grooves that are made each year. It is shown in the picture by four grooves travelling from the left side to the right side of the shell. The grooves are pointed out by four pairs of black arrows. The model that I'm making already has the many grooves going up and down represented by the yellow arrows. In order to finish my model, I'm trying to recreate the growth lines represented by the black arrows. I've tried different methods and none seemed to work. I just any sort of guidance created the growth lines. Help would be much appreciated.

SolidworksParts And Features