extinct genus of graptolites (small, aquatic colonial animals related to primitive chordates) found as fossils in Silurian marine rocks (formed 438 to 408 million years ago). The most common Silurian graptolite genus, Monograptus is characterized by a single branch, or stipe, in which distinctive features of the structure occur. Monograptus descended from the genus Diplograptus, a two-branched form. Several forms or species are known and are useful for correlating Silurian rocks in widely separated areas and for further subdividing Silurian time.