60 BETTISIT GRAPTOLITES. 



The tlicc;i3 have an average length of ;j'2 mm. ; 

 F,a.^^b.-T etraoraptusHcad i,UM. ^j^^^ ^^,^ -^^ coiitact fov rather more than three- 

 quarters their lengtli in tlie distal parts of the 

 stipes, but for rather less near the proximal end. 

 AJjihities. — T. EoacU resembles in general shape 

 Distal thoc^ North of Taifan, Whit- rj, qfuvhihrachiatu.^ and T. crucifcr, but the size of 



land. Coll. Woodwardian Mviseum. ^ -^ ' 



the stipes seems to distinguish it from the former, 

 while the characters of the thecal and the extreme shortness of the stipes of the 

 first order appear to separate it from the latter. But there can be no doubt that 

 2\ IL^adi and T. cntcifer are very closely allied, and they may possibly some day be 

 proved to be identical. 



Horizon and Localities. — Arenig, Middle Skiddaw Slates. 



Lal'p Disfn'cf : Randal Crag, Skiddaw; Barf, near Keswick. S. I^rolhiiit] : 

 Bennane Head, Ballantrae. S. Wah's .- North of Talfan, Whitland. 



Associates, etc. — This form occurs in the Skiddaw Slates, where it is associated 

 with Didymograjitus liirundo and D. gibber alus. According to the officers of H.M. 

 Geological Survey it has also been found in the Arenig rocks of South Scotland.^ 

 Good specimens have been met with in South Wales, associated with various 

 Dendrograpti, and are now in the Woodwardian Museum. The largest example 

 is from the Skiddaw Slates of Randal Crag, and is in Lapworth's collection. 



Tetragraptus Amii, Lapworth MS. Plate V, figs. 4 a — c. 



1865. Grajjtolifes hryonoides, Hall, G-rapt. of Quebec Grouji, pi. iv, figs. 9, 10. 



Main stipes from 2*5 to 5*5 cm. in length, approximately straight and rigid, 



slender at origin, l)ut widening rapidly to an average maximum of 3 mm. 



Thecse nine to ten in 10 mm., inclined at 45°, three to four times as long 



as wide, and free one quarter their length. Apertural margins normal, 



concave. 



Description. — The main stipes are, as a general rule, from 2*5 to 3*5 cm. long, 



though longer ones may occur. All show the characteristic rapid increase in width 



near their initial extremities, though some of the smaller specimens never attain 



the maximum breadth ; a width of 3 mm. being characteristic of the forms most 



commonly met with. All, however, show a slight diminution in breadth at their 



extreme distal end, owing to the incomplete growth of the thecffi at that point. 



The sicula has a length of about 2 mm., but it is only indistinctly indicated, 

 as a general rule, and details of the structure of the proximal end are likewise 

 obscure. 



' ' Memoirs of the Geol. Survey,' " Tlie Silurian llocks of Britain," vol. i, Scotland, p. 06]. 



