46 GUIDE TO THE FOSSIL INVERTEBEATE AMIHALS. 
Qallei’y X. observed to grow op from a sharply-pointed conical cup, the 
point of winch was sometimes attached to a long thread.' 
Such individuals may have been attached to some other 
animal or plant, l)ut cannot have been directly fixed to the 
sea-floor. The substance of these fossils is supposed to have 
been chitinous, like the periderm of Calyptoblastea, when the 
animals were alive; but actually it is not so. This fact 
and the absence of any representatives of the group from 
Devonian to Pleistocene times suggest that, though the 
Dendroidea may be Hydrozoa, still they cannot be closely 
related to the Calyptoblastea. 
Table-case Contemporaneous with the Dendroidea, and, some 
WaU^case tlerived therefrom, is the Order Graptolitoidea — 
6b, 0 . graptolites, a name given by Linnaeus from the likene.ss 
which the fossils bear to writing on the slates in which they 
are usually found compressed. The pointed conical chamber 
observ^ed in Dictyonenm is also characteristic of all grapto- 
lites, and is called the sicula (Fig. 19). In the growth of a 
Fig. 19. — An early form of Graptolite, Didynwgraptns uniformis, from the 
Ordovician of Britain. The sicula consists of a smooth embryonic 
part, and a later-formed part with growth-lines ; similar growth-lines 
occur on the thecae, but are omitted for the sake of clearness. Enlarged 
about 3 diameters. (Modified from Elies & Woods.) 
colony, a theca (with its contained polyp, be it always under- 
stood) budded out from the sicula ; and from this theca 
another budded ; and each of these by further budding gave 
rise to a long line of thecae connected by a common canal. 
xV single branch of such a simple form looks like the blade of 
a fret-saw, with a straight back, and the thecae forming the 
teeth, which are directed away from the sicular end. The 
sicula was almost certainly attached to something, and it is 
probable that in such forms as these it was attached to 
floating sea-weed ; sometimes it was fixed by a small disc- 
like expansion ; sometimes it hung by a long flexible thread. 
The earlier graptolite colonies branched many times, e.g. 
Bryograptus ; but the number of branches was gradually 
