suture are often useful in discriminating one species from another, for example, sometimes the suture is channeled. The suture also provides a sort of geographic marker from which one can refer to the positioning of patterning or sculpture, where that is relevant: for example some species have a darker or lighter subsutural band on the shell. When an angulation of the whorls occurs, the space between it and the suture above it (i.e. the abaxial edge of the sutural ramp) constitutes the area known as the "shoulder" of the shell. The shoulder angle may be simple or keeled, and may sometimes have nodes or spines In arthropods Sutures in the carapaces of trilobites A trilobite's carapace consisted of calcite and calcium phosphate deposited on a lattice (framework) of chitin (a polysaccharide). The trilobite body is divided into three major sections: a cephalon (head section) with eyes, mouthparts and sensory organs such as antennae; a thorax of multiple segments which are similar to each other; and a pygidium, or tail section. In many species the cephalon had sutures running from back to front round the outside edges of the eyes. These sutures divided the cephalon into 3 pieces. The sutures in trilobites' cephalons were unusual because it seems their main function was to create weaknesses which made it easy for this part of the carapace ("armor") to split when the animal needed to molt. References Skeletal system Animal anatomy Mollusc anatomy