The machaeridians are characterized by having serialized rows of calcitic shell plates. Modern-day scleractinian corals have skeletons composed of aragonite, a calcitic mineral. Periphyton is the basic building block of marl, a calcitic mud. Hederellids are extinct colonial animals with calcitic tubular branching exoskeletons. The Guane people chewed coca combined with calcitic grains, using poporos. These ash deposits hardened with time, creating layers of calcareous tuff and calcitic hard-pan soil. It is fine grained with sporadic calcitic fossil clasts. It also absorbs calcium from water, which adds to the calcitic composition of the marl. Where periphyton grows, the soil develops into marl, which is more calcitic in composition. The mineralogic composition of carbonate platforms may be either calcitic or aragonitic. It is a very monotonous sequence of calcitic sandstones and black schists. In Missouri, "Physaria filiformis" is a species restricted to calcitic limestone glades. The deposition of calcitic sediments is indicative of a climate with high evaporation rates. It is the only known scleractinian so far to have an entirely calcitic skeleton. The calcitic guard is the most common belemnite remain.