The Graptolites were compound animals, consisting of a number of polyps inserted in cups or thecae which budded out in a line from the primary sicula or conical chamber, which chamber was probably attached to floating sea-weed, either by a fine thread (nema), or a disc-like expansion. This budding of the polyp-bearing thecae gives to the polypary or colony the appearance of a fret-saw, with the teeth directed away from the sicula. The habit of the earlier graptolites was to branch repeatedly, as in _Clonograptus_, or to show a compound leaf-like structure as in _Phyllograptus_. Later on the many-branched forms had their branches reduced until, as in _Didymograptus_, there were only two branches. Sometimes the branches opened out to direct the thecae upwards, the better to procure their food supply. In _Diplograptus_ the thecae turned upwards and acquired a support by the formation of a medium rod (virgula), often ending in a disc or float. In Silurian times _Monograptus_ prevailed, a genus having only a single row of thecae supported by a straight or curved virgula. In _Retiolites_ the polypary opened out by means of a net-work of fine strands, rendering it better able to float, at the same time retaining its original strength. =Lower Ordovician Graptolites, Victoria.--= The Lower Ordovician slates and shales of Victoria have been successfully divided into several distinct series by means of the Graptolites. These, commencing at the oldest, are:-- (1) Lancefield Series. Characterised by _Bryograptus clarki_, _B. victoriae_, _Didymograptus pritchardi_, _D. taylori_ and _Tetragraptus decipiens_. Other forms less restricted are, _Clonograptus magnificus_ (measuring over a yard in breadth), _C. flexilis_, _C. rigidus_, _Leptograptus antiquus_ and _Tetragraptus approximatus_ (Fig. 73). (2) Bendigo Series. Characterised by _Tetragraptus fruticosus_, _T. pendens_, _Trichograptus fergusoni_ and _Goniograptus thureaui_. This series also contains _Tetragraptus serra_ (ranging into Darriwill Series), _T. bryonoides_, _T. quadribrachiatus_, _T. approximatus_ (base of the series), _Phyllograptus typus_, _Dichograptus octobrachiatus_, _Goniograptus macer_ and many _Didymograpti_, including _D. bifidus_ (Fig. 74). [Illustration: =Fig. 73.--LOWER ORDOVICIAN GRAPTOLITES.= A--Bryograptus clarki, T. S. Hall. L. Ordovician. Lancefield, Vict. B--Tetragraptus fruticosus, J. Hall sp. L. Ordovician. Lancefield. C--Phyllograptus typus, J. Hall. L. Ordovician. Lancefield. D--Goniograptus macer, T. S. Hall. L. Ordovician. Lancefield. E--Didymograptus caduceus, Salter. L. Ordovician. Lancefield. F--Trigonograptus wilkinsoni, T. S. Hall. L. Ordov. Darriwill, Vict. ] [Illustration: =Fig. 74.--LOWER ORDOVICIAN GRAPTOLITES.= A--Loganograptus logani, J. Hall sp. L. Ordov. Newham, Vict. B--Tetragraptus approximatus, Nich. L. Ordovician. Canada and Victoria. (_After Nicholson_)