See “Graphite and its Uses,” Bull. Imperial Institute, (1906) P. 353. (1907) p. 70; F. Cirkel, Graphite (Ottawa, 1907).
(W. G. M.)
GRAPTOLITES, an assemblage of extinct zoophytes whose skeletal remains are found in the Palaeozoic rocks, occasionally in great abundance. They are usually preserved as branching or unbranching carbonized bodies, tree-like, leaf-like or rod-like in shape, their edges regularly toothed or denticulated. Most frequently they occur lying on the bedding planes of black shales; less commonly they are met with in many other kinds of sediment, and when in limestone they may retain much of their original relief and admit of a detailed microscopic study.
Each Graptolite represents the common horny or chitinous investment or supporting structure of a colony of zooids, each tooth-like projection marking the position of the sheath or theca of an individual zooid. Some of the branching forms have a distinct outward resemblance to the polyparies of Sertularia and Plumularia among the recent Hydroida (Calyptoblastea); in none of the unbranching forms, however, is the similarity by any means close.
The Graptolite polyparies vary considerably in size: the majority range from 1 in. to about 6 in. in length; few examples have been met with having a length or more than 30 in.
Very different views have been held as to the systematic place and rank of the Graptolites. Linnaeus included them in his group of false fossils (Graptolithus = written stone). At one time they were referred by some to the Polyzoa (Bryozoa), and later, by almost general consent, to the Hydroida (Calyptoblastea) among the Hydrozoa (Hydromedusae). Of late years an opinion is gaining ground that they may be regarded as constituting collectively an independent phylum of their own (Graptolithina).
There are two main groups, or sub-phyla: the Graptoloidea or Graptolites proper, and the Dendroidea or tree-like Graptolites; the former is typified by the unbranched genus Monograptus and the latter by the many-branched genus Dendrograptus.
A Monograptus makes its first appearance as a minute dagger-like body (the sicula), which represents the flattened covering of the primary or embryonic zooid of the colony. This sicula, which had originally the shape of a hollow cone, is formed of two portions or regions—an upper and smaller (apical or embryonic) portion, marked by delicate longitudinal lines, and having a fine tabular thread (the nema) proceeding from its apex; and a lower (thecal or apertural) portion, marked by transverse lines of growth and widening in the direction of the mouth, the lip or apertural margin of which forms the broad end of the sicula. This margin is normally furnished with a perpendicular spine (virgella) and occasionally with two shorter lateral spines or lobes.
A bud is given off from the sicula at a variable distance along its length. From this bud is developed the first zooid and first serial theca of the colony. This theca grows in the direction of the apex of the sicula, to which it adheres by its dorsal wall. Thus while the mouth of the sicula is directed downwards, that of the first serial theca is pointed upwards, making a theoretical angle of about 180° with the direction of that of the sicula.