k 



DICHOGRAPTUS. 11 



(stipes of second order), whicli is also of short duration. Each of tlie four sti})es 

 thus formed is again dichotomously divided, with the result that eight stipes (stipes 

 of third order) are ultimately produced. These undergo no further division, and 

 persist as the eight main stipes of the polyparj. 



It is a matter for regret that the specimens from the Skiddaw Slates, the only 

 beds in Britain in Avhich this genus is abundantly represented, are, as a whole, 

 so poorly preserved. It is often impossible to make out any details concerning the 

 thecce, since the stipes are preserved with their dorsal surfaces uppermost, and with 

 the thecse embedded in the rock. This is almost invariably the case with the tliecae 

 on the stipes of the first and second order ; those on the stipes of the third order 

 (main stipes) are, however, often more or less clearly seen. 



It seems most probable that the main stipes of the Dichograpti grew in one and 

 the same horizontal plane. All the forms at present known fall naturally into one 

 horizontal series, which contains two groups. 



Horizontal Series.— GROur I. Type J). octoJ>racliiatiis. 



v. uctobracliiatits, 

 var. SedgtvicJcii. 

 GROur II. Type D. separatus. 



D. sejjaratas. 



Horizontal Series. 



Dichograpti in Avhicli the main stipes are straight or curved, and which grow in 

 one horizontal plane. 



Group I. — Type D. odobrachiatas. 



Dichograpti with robust stipes, having thecae which overlap for a considerable 

 portion of their length. 



Dichograptus octobrachiatus (Hall). Plates IX and X, figs. 1 a—e. 



1858. OmptoUthus octobrachiatus (Hall), Greol. Survey Canada Eep., 1857, p. 122. 



1863. DicJwgrapsus aranea (Salter), Quart. Journ. Geol. Soc, vol. xix, p. 137, figs. 9 and 10. 



1865. GraptoUthus octobrachiatus (Hall), "Ci-apt. of Quebec Grroup," Geol. Survey Canada, dec. 2, 



p. 96, lA. vii, figs. 1 — 7, and pi. viii, figs. 1 — 4. 

 1868. Bichograpsus octobrachiatus, Nicholson, Quart. Journ. Geol. Soc, vol. xxiv, p. 129, pi. v, figs. 



1 and 2. 

 1898. Dichograptus octobrachiatus, Elles (pars), Quart. Journ. Geol. Soc, vol. liv, p. 483. 



