46 GUIDE TO THE FOSSIL INVEETEBRATE ANIMALS. 



Gallery X. observed to grow up from a sharply-pointed conical cup, the 

 point of which was sometimes attached to a long thread. 

 Such individuals may have been attached to some other 

 animal or plant, but 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-ease Contemporaneous with the Dendroidea, and, some 

 waii^ca e ^^^PP*^^^' derived therefrom, is the Order Graptolitoidea — 

 6b 'c?^^ the graptolites, a name given by Linnaeus from the likeness 

 which the fossils bear to writing on the slates in which they 

 are usually found compressed. The pointed conical chamber 

 observed in Didyoncma 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, Didymograptus 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 Elles & 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. 

 A 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 



