Graptolites

Graptolites, a group of fossil hydrozoa, apparently related to the recent Sertularia. They had simple or branched polyparies, formed of a chitinous substance, and the polyparies were usually strengthened by a horny-like rod, which is called the 'solid axis.' Professor Nicholson thinks that the term 'solid' is probably a misnomer, and that the axis was most likely hollow and filled with living material. The cells in which the polypites lived were arranged in a single series on one side of the axis, or in a double series on both sides; the axis was generally prolonged beyond the cells at the growing end of the polypary. Reproductive buds, or ovarian vesicles, have been observed attached to the polypary, exhibiting a method of reproduction similar to that in the hydrozoa, but they differ from the ovarian vesicles of the modern Sertularians in becoming detached from the parent colony. The graptolites appear to have been free-floating organisms. They are generally divided into Monopronidian and Dipronidian groups. In the first named the polypary, whether single or branched, had only one row of cells, or 'hydrothecæ'; in the second the polypary was furnished with a row of cells on each side. The former group ranges from the base to the top of the Silurian system, while the latter is confined chiefly to the Lower Silurian. To this system the graptolites may be said to be confined. Numerous species have been described, and from their abundance in the argillaceous shales and greywackes it is obvious that they must have swarmed in the Silurian seas. There are several other Sertularian-like fossils often described as graptolites; such as Dendrograpus—a rooted plant-like form (Cambrian and Lower Silurian); Dictyonema, also plant-like, and probably rooted (Silurian); Retiolites, with no solid axis (Silurian), but otherwise resembling the graptolites.