18 Dr. 0. Herrmann — On the DichograjMdce — 



1. Pterograptus elegans, Holm, Tullberg. 



Graptolithus Murchisonii, Boeck ; G. gracilis, Kjerulf. 

 Primary branches diverging in curves at an angle of 50°-60°. 

 Didymograptus Murchisoni-zone (Et. 4), Norway ; zone with D. 

 Murchisoni ' geminus, Skane. 



2. Pterogaptus acutus (Hopk.), Holm. 



Ptilograptus acutus, Hopk. 

 Primary branches diverging at an angle of 30°-40°. 

 Lower Llandeilo, Wales. 



3. Pterograptus (?) dilaceratus, sp.n. (Woodcut, p. 17, Fig. 4.) 

 Hydrothecaa as in P. elegans, Holm. Secondary branches narrow 



and thin, never so rigid as in that species. Angle of divergence of 

 the two primary branches probably about 180°. The hydrosoma 

 is always fragmentary (sicula not seen attached). Not unfrequently 

 a primary branch is met with, from the hydrothecas of which closely 

 approximated branchlets project. It is sometimes rolled up circularly, 

 often only gently curved, or bent and broken into a zigzagged form. 



Genus VI. Pleurograptus, Nicholson. 



Secondary branches proceed from both sides of the primary branches, 

 and themselves again give off branches ; primary branches diverging 

 at an angle of about 180°. 



Eeferred by Lapworth to the Nemagraptidas. 2 



1. Pleurograptus vagans, Nich. 



Skiddaw Slates, N. England ; Phyllograptus-shales, Norway. 



2. Pleurograptus linearis (Carr.), Nich., Lapw. 



Cladograpsus linearis, Carruthers. 

 Upper Llandeilo, England ; Hartfell Shales, Scotland. 



3. Pleurograptus radiatus, Lapw. Hartfell Shales, Scotland. 

 The four-branched forms resulting from the dichotomous division 



of a Pidymograptus constitute 



Genus VII. Tetragraptus, Salter. 

 Hydrosoma bilaterally symmetrically developed, consisting of four 

 simple equivalent branches. A central disc may or may not be 

 present. 



The four-branched forms may be brought into two natural groups : 

 a. The two short branches issuing from the broad end of the 

 sicula diverge at an angle of about 180°, e.g. T. quadribracliiatus 

 (Hall). Thus is formed a funiculus uniting the two halves of the 

 hydrosoma, but this may be reduced until it nearly disappears {e.g. 

 T. Hicksii, Hopk., and T. Halli, Hopk.). 



/3. The two branches which originate from the sicula diverge at 

 an angle of ^180° (T. fructicosus (Hall)). 



Group a. 

 1. Tetragraptus quadribracliiatus (Hall), Nich., Ether., Hopk. 

 and Lapw., Linnars., Tornq., Brogg., Herrm., Tullb. 

 Graptolithus quadribracliiatus, Hall, M'Coy. 

 Tetragraptus crucialis, Salt. 



1 Phyllograptus-shdles,, Norway. 



2 See Notes on Rritish Graptolithes, by C. Lapworth, F.G.S., Geol. Mag. 1873, 

 Vol. X. p. 558, and Table I. 



