SUB-CLASS I HY DROZOA—GRAPTOLITOIDEA aLz/ 
vesicle, or a filiform proximal virgula, or again into two spiniform or spur-like 
processes. Sometimes the hydrosoma remains undivided, sometimes it forms 
branches, which may diverge at various angles; in other cases two or four 
monoprionidian polyparies may be placed back to back with their dorsal walls 
coalescing, thus giving rise to di- or tetra-prionidian colonies. In the latter 
types the coenosark is commonly divided by one or two median septa, and 
the primordial buds originate at various distances along the sicular margin. 
Occasional specimens of diprionidian Graptolites have been found bearing 
large sac-like or irregularly expanded corneous structures (Fig. 205), which are 
comparable with the ovarian capsules (gonangia) of recent Sertularians. Further- 
more, the singular bodies known as Dawsonta—small, oval, bell-shaped, or 
conical chitinous capsules, which are often associated with Graptolites in vast 
numbers—are regarded by Nicholson as detached graptolitic gonangia. 
Graptolites commonly occur in argillaceous schists, more rarely in limestone 
formations, of the Upper Cambrian, Ordovician, and Silurian systems. They 
seem to have swarmed in the muddier portions of the sea, and floated either as 
free-swimming colonies, or, in rarer instances, remained stationary with the 
sicula embedded in the mud. They are divided into three groups :—Jono- 
prionidae, Diprionidae, and Retiolitidae. 

Sub-Order A. MONOPRIONIDAE. Hopkinson. 
Hydrothecae uniserially developed on the side of polypary lying opposite to the 
virgula. 
Monograptus, Gein. (Monoprion, Barr.; Pomatograptus and Pristiograptus, 
Jaekel), (Figs. 202, 206). Hydrosoma undivided, rectilinear, or curved, some- 
a b ec 
| 

Fic. 208. 
Coenograptus gracilis, Hall. Ordo- 
vician: Point Lévis, Canada (after 
Hall). 
Fic. 207. 
Rastrites Linnaei, Barr. 
Fic. 206. Silurian; Zekkowitz, near 
r. 
Prague (after Barrande). 
* a, Monograptus Nilssoni, 
Barr. Silurian(Alumn Schists); 
Grafenwerth, near Schleitz, 
Germany; 06, Monograptus 
colonus, Barr. Silurian ; 
Eliotstield, Scotland. Show- 
ing sicula (after Lapworth); 

Fic. 210. 
c, Monograptus turriculatus, Fia. 209. 
Barr. Silurian; Prague (after Didymograptus pennatulus, Hall. Tetragraptus bryonoides, Hall. 
Barrande). Allfigures natural Ordovician; Point Levis, Canada Ordovician ; Point Lévis, Canada 
size. (after Hall). (after Hall). 
times helicoid. Hydrothecae in contact, usually overlapping. Aperture either 
entire or contracted, often directed downwards. Abundant from base to 
summit of Silurian. 
