CLASS II 



HYDEOZOA— GRAPTOLITOIDEA 



129 



Order 2. GRAPTOLOIDEA Lapworth. 

 Suborder A. AXONOLIPA Frech (emend. Ruedemann). 

 Family 1. Dichograptidae Lapworth. 

 Uniserial Graptolitoidea tvith bilateral rhabdosome ; brauch es dichotomous ; thecae 



simple^ sub-cylindrical. 



Dichograptus Salter (Fig. 198). 



Rhabdosome consisting of eight simple 



Fia. 199. 



Tetragraptus hryonoides Hall. 

 Ordovician ; Point L6vis, Canada 

 (after Hall). 



Fig. 200 



Didymograptus pennatulus Hall. 

 Ordovician ; Point L^vis, Canada 

 (after Hall). 



Fig. 198. 



Dichograptus octobracMatus Hall. Ordovician (Quebec 

 Group) ; Point L^vis, Canada (after Hall). 



Fio. 202. 



Didyynograp' l'hyllograjnus typv,n Hall. 

 t)is murchisoni Ordovician (Quebec Group); 

 Point L^vis, Canada. a, Several 

 polyparies of the natural size ; 

 b, Ideal cross-section, enlarged 

 (after HaH). 



(Beck). Middle 

 Ordovician 

 (L 1 a n d e i 1 

 Group) ; Wales. 



uniserial branches which are produced by repeated dichotomy, and their bases 

 often enveloped in a central corneous disk. Ordovician. 



Tetragraptus Salter (Fig. 199). Rhabdosome consisting of four uniserial 

 branches which are produced by twice repeated dichotomous division. 

 Ordovician. 



Didymograptus M'Coy (Figs. 195, J; 200; 201). Rhabdosome consisting of 

 two symmetrical branches diverging from a small primary cell (sicula) at 

 various angles. Thecae obliquely directed, having the form of flattened 

 rectangular prisms, and in contact with one another throughout. Ordovician. 



Phyllograptus Hall (Fig. 202). Rhabdosome leaf-like, composed of four 



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