

74 BRITISH GRAPTOLITES. 



Dt'.^n'iptioii. — The lateral branching; traced aloii,^ one or other of the main stipes 

 is seen to take place approximately at intervals of 25, 82, and 38 mm. respectively. 

 The angle at Avhich the lateral branches come off varies from abont 90° to G5°. 



From the lateral branches additional lateral 



Pia. iS.—Trochograptus diffusus, Holm. , . „„ 



branches may be given oft. 



A small square disc with an average diameter of 

 about (3"3 mm. commonly encloses the proximal parts 

 of the main stipes, but this may be absent. 



Ajfiiiitie!^. — We have here referred the British 

 examples of Trocliogvaptus to the only species 

 hitherto described, ?". e. T. diffiisns, Holm (loc. cit.) ; 

 thouo-h they differ from it somewhat in havino' a more 



Distal thocoe on one of the Lateral o ./ ^ o ^ 



branches of specimen figured on irresTular mode of branching', and more thecae in a 



PI. VIII, ficf. 1 a. ^ °' 



given unit of length. T. diffusus resembles other 

 compound Tetragraptids, such as Schizog. reticulatus, S. tardifnrcatus, and Holog. 

 Drcnti, in general plan of growth. The compound lateral branches, however, 

 seem to distinguish it from any Hchizognipfas, while from Ilolog. Deaiii it may 

 be separated by the fact that the lateral branches are all on the same side of 

 the main stipe, while in H. Deaiii they are apparently given off at irregular 

 intervals on both sides. 



Horizon and Localities. — Arenig, Middle Skiddaw Slates. 



Lalce District : Scaw Gill ; New BroAV Quarry, Upper Lorton ; Whit Beck, 

 Upper Lorton ; Grisedale Pike. 



Associates, etc.—^T. diffusus is only known in Britain from the Skiddaw Slates, 

 where large specimens occur at certain horizons in some abundance. Its associates 

 are not known. 



There are good specimens in the collections of the Keswick Museum and the 

 Woodwardian Museum. 



Genus HOLOGRAPTUS, TMm. 



1881. Holograplus, Holm, " Tvenne nya sliigteu af familjen Dieliograptidno," Ofv. Konol. Vet. Akad. 



Stoclcholm Fijrh., vol. xxxviii, No. 9, p. 45. 

 1893. BfmvilUgrapfus, Barrois, Ann. d. 1. Soc. Geol. du Nord, pi. x.xi, p. 107. 



Polypary robust, typically bilaterally symmetrical, consisting of four main 

 stipes many centimetres in length, produced by dichotomous division 

 from a primitive Didymograptiis stage. From both sides of each of these 

 four main stipes many lateral branches, several centimetres in length, are 

 given off at subregular intervals; some of these lateral branches are 

 simple, others compound. 



