I sketched another shell tonight. Playing with the darks in the crevices like Katherine suggested. I think I got more depth but my spirals were a little uneven. Also the skell doesn't look much like the original. It is not as dark as the sketch. I will play around with it some more and see if I can get better with it. Both were done with a mechanical pencil. The one in the former post was colored with dry watercolor pencils. I am also including a picture of the shell.