Mulholland and Rapp independently propose a basic classification for phytoliths but focus on grass silica cells. Each type of cell that becomes silicified (e.g., trichome, stomata, etc.) is distinguished as a category. Silica cell phytoliths are further subdivided into eight morphological forms. These shapes are subdivided by cross-sectional and three-dimensional characteristics (crenulations, depressions, and concavities), distinguishing as many as 111 forms. The degree of detail used depends partly on the nature of the specific problem. When working on a broad subfamily basis the eight-fold classification is often sufficient, but work at a finer scale requires measurement of more parameters. They present the basic components of their classification system in this article.