SUB-CLASS I HY DROZOA—GRAPTOLITOIDEA 115 
which have been considered by various authors as plant remains, horny sponges, 
Pennatulidae, Cephalopods, and Br yozoans. Portlock, in 1843, first pointed out 
their analogy with the Sertularians and Plumularians ; and his inferences as to 
their common relationship were gradually corroborated by the painstaking 
researches of Allman, Hall, Hopkinson, Lapworth, Nicholson, and others. The 
Graptolites differ, however, from all existing Hydromedusae, and also from the 
closely related Cladophora, in the fact of their non-attachment, and in that a 
rod-like axis is almost invariably developed in the periderm. 
Graptolites are generally found in an imperfect state of preservation, lying 
flattened in the same plane upon the slaty laminae in which they are embedded, 
and associated in large numbers. More rarely they occur in limestone, when 
the internal cavities are filled with calcareous matter, and the original form 
accurately preserved. 
The general skeletal tissue (periderm) was obviously flexible, and composed of 
smooth or finely striated chitine ; usually it has the form of a dense continuous 
membrane, but in the Fetzolitidae it is attenuated and supported by a latticed 
network of chitinous threads. It is usually preserved as a thin bitumino- 
carbonaceous film, which, however, is often infiltrated with pyrites, and is not 
infrequently replaced by a glistening greenish-white silicate (Giimbelite). 
The organism or hydrosoma of the Graptolites is usually linear, more rarely 
petaloid in form, undivided or branching, and is either straight, bent, or in 
exceptional instances spirally enrolled. Cup-shaped hydrothecae, which are 
usually obliquely set and more or less overlapping, are borne on one or on 
both sides of the polypary, and are united by a common coenosarcal canal 
enclosed in the periderm. The polypary is strengthened by a peculiar chitinous 
axis (virgula, solid axis), which in the monoprionidian forms runs in a groove 
lying outside the coenosark on the dorsal side of the organism (7.e. on the side 
opposite to the polypiferous margin). But in the biserial Graptolites the 
virgula is generally double, and the two halves are either enclosed between the 
laminae of a central or sub-central septum, which is formed by the coalescence 
of the flattened dorsal walls (Diprionidae) ; or they are placed on opposite sides 
of the coenosark, and are united with the peridermal network (Letiolitidae). 
Very commonly the virgula projects at one or at both extremities, but notably 
at the distal end of the polypary, as a longer or shorter naked filament : its 
proximal extension is often called the radicle. 
Springing from the common canal, which runs parallel with the virgula, is 
a series of hydrothecae (thecae, cellules, denticles), which are disposed in longi- 
tudinal rows along either one (Fig. 202), two (Fig. 203), or four sides of the 
polypary. They usually have the form of elongated, cylindrical, rectangular, or 
conical sacs ; their walls are in most cases applied to those of their neighbours 
above and below, although occasionally they spring out quite isolated from 
one another. Each hydrotheca opens directly into the common canal, and is 
furnished distally with an external aperture, the form and size of which vary 
in Quart. Journ. Geol. Soc., 1875, 1878, 1881, and in Annals and Mag. Nat. Hist. 1879, 1880.— 
Nicholson, H. A., Monograph of the British Graptolitidae, 1872.—Perner, J., Etudes sur les Gray 
tolites de Bohéme, 1894.— Richter, R., Thiiringische Graptolithen (Zeitschr. deutsch. geol. Gesellsch.), 
Bd. V., 1853 ; XVIII., 1866 ; and XXIII, 1871.—Scharenberg, W., Ueber Graptolithen. Breslau, 
1851. Suess, E., Ueber bohmische Graptolithen (Haidinger’s Naturw. Abhandl. Bd. IV. Abth. 
I.), 1851.—Térnquist, S. L., Observations on Graptolites (Acta Univ. Lund. XXVIL, XXVIIL., 
XXIX.), 1890-92.— Wiman, C., Ueber Monograptus und Diplograptidae (Bull. Geol. Inst. Upsala, 
I.), 1893. [Translated in Journ. Geol. vol. II. p. 267, 1893.] 
