14 Dr. 0. Herrmann — On the D'whograplidoe — 



1. Didymograptus, M'Coy (belonging to Lapworth's family Dicho- 



graptidae) with such forms as D. Murchisoni, Beck and 

 D. vacillans, Tullb. (Woodcut, p. 13, Fig. 1). 



2. Dicellograptus, Hopk. (of the family Dichograptidae, Lapw.) 



with such forms as D. Forchhammeri, Gein. (Fig. 2). 



3. Dicranograptus, Hall (of the family Dicranograptidaa, Lapw.) 



with such forms as D. ramosus, Hall (Fig. 3). 

 To these may be added the still somewhat problematical genus of 

 the family Dichograptida? : — 



4. Janograptus, Tullb., with such forms as Janograptus laxatus, 



Tullb. (see p. 16). 

 The following genera are adopted by the author : — 



Genus I. Didymograptus, M'Coy. 



Hydrosoma bilaterally symmetrically developed, consisting of two 

 simple branches diverging at an angle of 70° — 180°. Sicula well 

 preserved and always visible. The two primordial buds originate in 

 the neighbourhood of the broad end of the sicula. Lower Silurian. 



Species. 

 a. Type Didymograptus patulus, Hall. 



1. Didymograptus patulus (Hall), Nich., Hopk. and Lapw., Brogger. 



Graptolithus patulus, Hall, D. hirundo, Salt., Nich. 



Angle of divergence about 180°. 



Horizons and Localities. — Quebec Group, Canada; Middle and Upper 

 Arenig, St. Davids ; Skiddaw Slates, North of England ; lower part 

 of the Fhyllograptus-skeiles, Norway ; Pliyllograptiis-sha\es, Skane 

 and West Gothland. 



2. Didymograptus constrictus (Hall), Linnars., Lapw., Brb'gg. 



Graptolithus constrictus, Hall. 



Quebec Group, Canada ; Lower Graptolite shales, Skane ; Pliyllo- 

 graptus-sh&les, Norway. 



3. Didymograptus extensus (Hall), Nich., Hopk., and Lapw., M'Coy, 



Brogg. 



Graptolithus extensus, Hall. 



Quebec Group, Canada ; Lower Arenig (Wales) ; doubtfully in 

 the Lower Bala of Australia; Lower part of Phyllograptus-sbiedes, 

 Norway. 



4. Didymograptus euodus, Lapw. (and Hopk.). 



D. extensus, var. euodus, Lapw. 

 Hor. and Loc. — Lower Llandeilo, Wales. 



5. Didymograptus arcuatus (Hall). 



Graptolithus arcuatus, Hall. 

 Quebec Group, Canada. 



6. Didymograptus extenuatus (Hall), Lapw. 



Graptolithus extenuatus, Hall. 

 Quebec Group, Canada. 



7. Didymograptus similis (Hall), Lapw. 



Graptolithus similis, Hall. 



Quebec Group, Canada. 



