20 Br. 0. Herrmann — On the Dichograptidce — 



From Tetragraptus two different groups of forms may be derived. 

 The four branches may divide dichotomously, producing hydrosomata 

 with eight or more branches (Dichograptus, Salt.) ; or secondary 

 branches may shoot forth from the four primary branches upon one 

 or both sides. The latter is the case in the following five genera. 



Genus VIII. Schizograptus, Nicholson. 



Hydrosoma bilaterally subsymmetrical, composed of four primary 

 branches, which issue in the form of a cross from the starting-point. 

 Each of these branches developes a series of rigid branchlets which 

 originate on each primary branch from the same side, and apparently 

 do not again divide. Hydrothecas as in Monograptus Sagittarius, His. 

 No central disc. 



1. Schizograptus reticulatus, Nich., Lapw. 

 Dichograptus reticulatus, Nich. 



Lower part of Skiddaw Slates, N. England. 



Genus IX. Trochogbaptus, Holm (Ofv. Kongl. Vet. Akad. Forh. 



1881). 



Constructed exactly like the preceding genus, except that the 

 secondary branches give off branchlets always upon the same side. 



1. Trochograptus diffusus, Holm, Brogger. 



Phyllograptus-shales (Et. 3, b), Norway. 



Genus X. Ctenograptus, Nicholson. 



This genus, provisionally established by Nicholson, probably 

 belongs to this group. The fragment figured by Nicholson resembles 

 a portion of Trochograptus diffusus, figured by Holm, except that the 

 hydrothecse appear to be essentially different. 

 1. Ctenograptus annulatus, Nich. 

 Diehograpsus ? annulatus, Nich. 

 Lower part of Skiddaw Slates, N. England. 

 Genus XI. Provisional. 



1. Graptolithus Bichardsoni, Hall, from the Quebec Group. The 

 mode of division of the branches as shown by the fragment figured 

 resembles that of Ctenograptus. But the commencement of the 

 hydrosoma is unknown, and we have no knowledge of the number 

 of primary branches. 



Genus XII. Holograptus, Holm (loc. cit.). 



Hydrosoma bilaterally subsymmetrical. From the four primary 

 branches branchlets are given off on both sides at irregular intervals. 

 Hydrothecse as in Didymograptus. 



1. Holograptus expansus, Holm. 



Phyllograptus-sha,\es, West Gothland. 



Genus XIII. Goniograptus, M'Coy (Prodr. Pal. Vict. dec. v.) 



1. Goniograptus Thureaui, M'Coy, Lapw. 

 Arenig, Australia. 



These smaller genera scarcely contain more than a single species 

 each, and probably in their case the establishment of genera has been 



