SUM-CLASS i HYDKOZOA < :KA 1TOLITOIDEA 115 



which haw Wen considered l.y various authors as plant iv ma ins, horny sponges, 

 Pennatulidae, (Vphalopods. and Bryozoans. Portlock, in is i:;, first pointed out 

 their analogy witli the Scrtularians and Plumularians ; and liis inferences as to 

 their common relationship were gradually corroborated by the painstaking 

 researches of Allmaii. Hall, llopkinson, Lapworth, Nicholson, and others. The 

 Uraptolites ditl'er. however, from all existing HydromeduSM, and also from the 

 closely related ('/mlnjilmrti, 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 

 >mooth or finely striated chitine ; usually it has the form of a dense continuous 

 membrane, but in the Retiolitidae it is attenuated and supported by a latticed 

 network of chitinous threads. It is usually preserved as a thin bitumino- 

 carbonaeeous film, which, however, is often infiltrated with pyrites, and is not 

 intVeijui'iitly replaced by a 'glistening greenish-white silicate (Giimbelite). 



The organism or hydrosoma of the Graptolites is usually linear, more rarely 

 jictaloid 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 on,e 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 (rir'iii/ii, solid axis), which in the monoprionidian forms runs in a groove 

 lying outside the coenosark on the dorsal side of the organism (i.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 (Rdiolitidae). 

 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 witn the virgula, is 

 a series of hydrothecae (th<'i-<n; c<'llules, 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 

 abo\e and below, although occasionally they spring out quite isolated from 

 one another. Kach hydrotheca opens directly into the common canal, and is 

 furnished distally with an- external aperture, the form and size of which vary 



in Quart. .Fc.inn. CJeol. Soc., 1875, 1878, 1881, and in Annals and Mag. Nat, Hist. 1879, 1880. 

 Xi,-hnl*u,,, 11. .!., Monograph of the British (Jraptolitidai-, 1872. ]',;.,-, ./., Etudes sur les Grap- 

 I"li1rs dt- Bohrme, 1894. Richter, Jl., Thiinn^ische (Jraptolithrn (Zritsrhr. dciitscli. ^i-ol. (u-srllsch.), 

 lid. V.. is:,:; : A VI 1 1., 18(56 : and XXIII., lS71.Scharenbenj, W., Uel.cr Graptolithen. Bn-shm, 

 1851. SUM, A'.. [Jebet btihmiache (Ji-aptnlitht-ii (Haitlii.^-r's Natunv. Al.liandl. Bd. IV. AUtli. 

 I.), 1851.Tomquisf, S. ., Ol.s.-rviition.s on Graptolites (Acta Univ. Lund. XXVI.. XXVIII.. 

 XXIX.), 1890-92. Wiman, 0., Ueber Monograptus und Diplograptidae (Bull. Geol. lust. Up.su la. 

 I.), 1893. [Translated in Journ. Geol. vol. II. p. 267, 1893.] 



