This beautiful, high-definition sea urchin shell can be printed in colour or black and white, giving you the choice of pencil or coloured pencil. It will advance your students drawing skills as there is a good range of tones/values and it has lots of detail. This would work well in sea shell and natural forms projects.
There are two versions of this one-page resource. One with the grid lines in place, and one which has little marks on the edge so your students can draw their own grid lines.