GRAPTOLITOIDEA. 



211 



A c) a bud, constituting the first hydrotheca, appears on one side, 

 near the thicker end of the " sicula." If the form under observation 

 belongs to the group of Graptolites in which the polypary possesses 

 two rows of hydrotheca^ (the " diprionidian " Graptolites), a second 

 bud next appears on the opposite side of the " sicula " to the first 

 one, and a little in advance of it. In the further development of the 

 polypary, the sicula appears to remain stationary, usually persisting 



Fig. 92. — A, Early stages in the development of Graptolites : a, The earliest stage of the 

 "sicula," in which no virgula is developed ; b, Later stage, with a virgula ; c, Still later stage, 

 with a single primordial hydrotheca; d, Stage with two hydrothecae. b, Base of Monograptus 

 grcgarius, enlarged ; C, Monograptus Clingani, young specimen, of the natural size, showing a 

 proximal and distal extension of the virgula ; d, Proximal portion of Didymograptus Murchisoni ; 

 E, Proximal portion of Climacograptus normalis, enlarged ; F, Proximal portion of Di)norpho- 

 graptus, enlarged. .? Sicula ; v Virgula. All the specimens are Ordovician or Silurian. (Partly 

 after Lopworth and Tullberg, and partly original.) 



at the base of the organism as a dagger-shaped process, the broad 

 end of which is directed proximally (fig. 92, b — f). In some cases 

 the sicula becomes obsolete, or it may ultimately develop into a 

 branch, or, rarely, into a vesicle (as in Diplograptus physophord). 



Taking a simple Graptolite, such as Monograptus priodon (fig. 93), 

 as the type of the sub-class, the polypary will be seen to consist 

 of three principal morphological elements — viz., the coenosarc, the 

 hydrothecae, and the virgula. The last mentioned of these — often 

 spoken of as the " axis " — is one of the most characteristic of the 

 structures of the Graptolitic polypary, and has the form of a cylin- 

 drical fibre or rod, probably hollow internally, which gives support 

 to the flexible chitinous skeleton. It runs, in such types as Mono- 

 graptus, along the dorsal side of the polypary, opposite to the hydro- 



