diprionidian

— Two leading types may be distinguished amongst the Graptolites, which are termed respectively "monoprionidian" and "diprionidian." The monoprionidian Graptolites, such as Monograptiis priodon (fig. 54), are distinguished by the fact that the polypary, whether simple or branched, possesses but a single row of cellules or "hydrothecæ." In the diprionidian forms, on the other hand, as in Diplograptus (fig. 63), the polypary possesses a row of cellules on each side.