128 



COELENTERATA— HYDROZOA 



PHYLUM II 



graptus and Ptilograptns. Their polyparies are finely branching and plant-like 

 in appearance, sometimes furnished with a strong foot-stalk, in other cases 

 terminating acutely at the base. The original substance was undoubtedly 

 chitinous. In well-preserved specimens are seen on one or occasionally on 

 both sides of the branchlets numerous small cellules or thecae, in which 

 evidently the zooids were seated. These thecae have been shown by Wiman 

 to be of threefold character, some of them having lodged nourishing, others 

 budding and others sexual individuals or goiiangia. Very often the branches 

 of the dendroid rhabdosome are united by numerous delicate processes or 

 dissepiments. 



Dendrograptus Hall. Rhabdosome consisting of a strong main stem and a 

 broad, spreading, shrublike, variously ramifying frond. Thecae commonly 



i 



Fio. 197. 



Dldyonemii fldheUi forme (Eichw.). Upper Cambrian; Rensselaer County, N.Y. 

 neina, x -Vi- i', Mature rhabdosome with adhesion stem, x i/i (after Ruedeniann). 



A, Sicula with very long 



obscure, but sometimes distinct and angular, or they may occur as round or 

 elliptic pits or pustules. Cambrian to Silurian. 



Ftilograptus Hall. Rhabdosome with branches giving off" branchlets 

 alternately on opposite sides, the general habit being suggestive of Recent 

 hydrozoans. Ordovician and Silurian ; eastern North America. 



Dirfyonona Hall {Didyograptus Hopkinson) (Figs. 196, 197). Rhabdosomes 

 forming funnel or fan-shaped fronds, composed of numerous bifurcating 

 branches arising from an acute base, and united at intervals by fine dissepi- 

 ments. Thecae with complicated appendages, their branches supporting three 

 kinds of individuals, nourishing, budding and sexual. Cambrian to Car- 

 boniferous ; especially abundant in Ordovician of Norway, but usually com- 

 pressed into a basket-like network. 



Desmograptus Hopkin. Dift'ers from the preceding in the flexuous char- 

 acter of the branches, which coalesce at intervals ; dissepiments chiefly in 

 lower part of the frond. Ordovician to Devonian ; Europe and North 

 America. 



