The latter part of the Ordovician saw a turnover in the graptoloids, the planktonic branch of the early Palaeozoic colonial graptolites (earlier posts on graptolites can be found here and here). The basally-branched forms that had been among the earliest graptoloids became extinct, leaving only the scandent forms surviving in the Silurian. Scandent colonies grew as a single linear stipe, which might have thecae (the openings for individual members of the colony) in two lines, one on either side of the stipe, or in a single line on one side of the stipe. The latter form characterised the Monograptidae, which were to become the latest surviving graptoloids (Bulman 1970).
Despite their long survival, monograptids were mostly a fairly uniform bunch. Their simple morphology did not give much scope for obvious morphological variation, and many monograptid lineages were not greatly distinct in colony form. One of the more distinct forms was the short-lived genus Rastrites which occurred during the Llandovery period of the early Silurian (and so relatively early in the history of the monograptids). Species of Rastrites developed very long and slender, widely spaced thecae that stode out perpendicular from the connecting stipe like the strings on an Incan quipu.
Rickards et al. (1977) suggested that the genus Rastrites is polyphyletic, deriving separate lineages within the genus from a number of different (albeit closely related) species assigned to the separate genus Monograptus. To a certain extent, the distinction is a little arbitrary: these ancestral Monograptus (treated by some authors as a third genus, 'Demirastrites') also had elongate thecae, but not yet to the same degree as developed by Rastrites proper, and not as widely spaced. But that, after all, is the nature of evolution, and the limitation of any classificatory system. You have to draw a line somewhere.
Bulman, O. M. B. 1970. Graptolithina with sections of Enteropneusta and Pterobranchia. In Treatise on Invertebrate Paleontology Part V 2nd ed. (C. Teichert, ed.) pp. V1-V149. The Geological Society of America, Inc.: Boulder (Colorado), and the University of Kansas: Lawrence (Kansas).
Rickards, R. B., J. E. Hutt & W. B. N. Berry. 1977. Evolution of the Silurian and Devonian graptoloids. Bulletin of the British Museum (Natural History): Geology series 28 (1): 1-120.