TETRAGRAPTUS. 



G9 



Fig. 4:2 a. — Tetnujraptus Bvjshyi, Hall. 



-.%/ 





Proximal end, preserved as an impres- 

 sion. Enlargement of PL VI, fig. G h. 



Fio. i'Zh.— Tdraijraidus Bigshyi, Hall. 



The siciila is about 2"3 mm. in lengtli, and tlio })roximal part of the ])olypary 

 has a similar structure to that of B. gibberulus ; but 

 whereas tlie earliest thecse in that species are longer 

 than any of those subsequently developed, those first 

 developed in T. IJigsbyi are the shortest. Holm's 

 work (loc. cit.) on the structure of this species 

 goes to show that the earliest theca arises from the 

 sicula near its apex, and the length of the two stipes 

 of the first order (" funiele ") is reduced to a niiiiinium, for dichotomous division 

 appears to take place within the earliest formed thecas, th. 1^ and th. 1~. 



The thecEe are at first nearly at right angles to 

 the general direction of the main stipes ; those 

 developed later have an initial angle of 40° — 50°, but 

 curve so that near its aperture the theca has ai/ 

 inclination of 60° — 70° to the general direction of 

 the stipe. The theca? are widest at their apertures, 

 and the apertural angle is markedly obtuse after 

 compression, averaging about 140° with the general 

 direction of the stipe. 



Affudties.- — T. Bvjslijl is easily recognisable as a 

 general rule, as it is unlike any other known form of 

 Tetragraptus, but when preserved so as to reveal two stipes only, it seems often 

 to have been confused with D. gibhendus. These two forms should, however, be 

 readily separated, since the stipes of T. Bigsbyl are narrowest at their proximal 

 ends, while those of D. gibberulus are ividest in that region. 



Horizon and Localities. — Arenig, Lower and Middle Skiddaw Slates. 

 Lake District: Randal Crag, Skiddaw; Gibraltar; White House Fell, Skiddaw ; 

 Bassenthwaite Sand-beds; Outerside; Troutbeck. S. Scotland: Bennane Head, 

 Ballantrae. 



Associates, etc. — T. Bigsbijl occurs in abundance in the lower parts of the 

 SkiddaAV Slates, and has been found at Barf, associated with Br yog. Kjcnilfi, var. 

 Cfimbrensis, and at Outerside with B. Nicholsoni. It is also recorded by the 

 Geological Survey (loc. cit.) from the Arenig rocks of Ballantrae. 



Numerous specimens are in the collection of the Woodwardian Museum, the 

 Keswick Museum, and in Postlethwaite's and Lapworth's private collections. 



Si 



Distal tliccae, preserved as an impression 

 Enlargement of PI. VI, fig. Gh. 



