FIGURE 2. General structure and features of the lower first molar in arvicoline rodents showing position of primary and secondary wings. Individual triangles are referred to by a capital "T" and a unique number (e.g., T1). Enamel closure between T1 and T2 depicts a closed triangle state. Confluence of T6 and T7 depicts an open triangle state. Modified from Bell and Jass (2004).