7Q BRITISH CxRAPTOLITES. 



is given off from tlio outer curve of the main stipes tlian from tlie inner, and it is 

 usually impossible to say on which side a branch arose originally. 



As a rule, little more is preserved than the general outline of the branches, the 

 apertures of the thecfie being usually em])edded in the rock. A true profile view is 

 rarely seen, so that the characters of the thecoe cannot all be made out. 



Affinities. — The British example of Holographis figured above, which is 

 distinguished as Holograjytas Deani in Lapworth's collection, closely resembles 

 BouvlUigraptus {Hologvaptus) Rickardsoni of Barrois (Inc. cit. supra) in general 

 appearance. It differs, however, in the fact that the thecse are more closely set 

 (nine as against seven to eight in 10 mm.), and the point of origin of the first of 

 the lateral branches is more than twice as distant as in the French form. Too 

 little is known of Hall's original species, G. Btchardsonl (' Grapt. of the Quebec 

 Group,' 18G5, p. 107, pi. xii, figs. 1, 8), to be certain of its specific identity or 

 otherwise. Hologra/ptus Deani appears to differ from //. cxpansus, Holm, in mode 

 of branching, and in the more closely set thecse (nine as against seven to seven 

 and a half in 10 mm.). The American, Swedish, and British examples are all, 

 however, closely related to each other, and although at present they are separated 

 as distinct forms, it is not impossible that they are variants of one and the same 

 widely distributed species. 



Horizon and Localitij. — Arenig, Skiddaw Slates. 



LaJce District : Newlands, near Buttermere. 



Associates. — Holograjptus Deani occurs in the Skiddaw Slates associated with 

 Tetrog. quadrihrachiatus, Didymog. v-fracivs, and D. Nicholsoni. 



The only specimen is in Lapworth's collection, and is called after its discoverer, 

 the late Dr. Deane, of Birmingham. 



Genus DICHOGRAPTUS, Salter (pars). 



18G3. DicJiograpsvs!, Salter (j)ars), "Note on the Skiddaw Slate Fossils," Quart. Joiirn. Geol. Soc. 

 vol. xix, p. 137. 



Folyparij typically bilaterally symmetrical, consisting of eight long uniserial 



main stipes, which are produced l)y repeated dichotomous division through 



Didymogra/ptus and Tetragraptus stages. 



Thecae simple, cylindrical or subcylindrical tubes, inclination and ovei-lap 



varied. 



The polypary is characterised by the possession of stii)es of three orders only. 



It would seem to have originated on lines similar to those indicated for Tetragraptus ; 



but in this case in addition to the primary Didymograptns stage (stipes of first 



order), which is short-lived, there is superadded a secondary Teiragraptus stage 



