58 BEITISH GRAPTOTJTES. 



Jjjinifii'.'i. — 7'. qiuidrihyiichiiifufi is a cliaracteristically slender species; it may be 

 regarded as allied in general form to 7'. Ifcudl and T.crucifer ; indeed, it is probable 

 that many of the larger and wider forms often erroneonsly referred to T. quadvl- 

 hrachiatus should be relegated to T. cnicifer, from, which T. quadribrachiatas differs 

 chiefly in the width of its stipes. It is distinguished from T. Headi in the 

 character of the proximal end and l)y the form of the thecge. The presence or 

 absence of a disc, in our opinion, is not a character of specific importance. 



Horizon and LocaUti('s. — Middle Arenig, Middle Skiddaw Slates (Middle and 

 Upper beds). 



LaJie District : Barf ; Outerside ; Bassenthwaite Common ; Randal Crag, 

 Skiddaw; Carlside Edge, Skiddaw ; Scawgill; Scale Hill, Crummuck ; N.E. 

 Grisedale Pike; Raven Gill. I^t. David's District: Whitesand Bay. ;S'. Scotland : 

 Bennane Head, Ballantrae. 



Associates, etc. — T. qnadi'ihrachiatus is found in the Skiddaw Slates associated 

 with T. crucifer, Didymog. Nicholsoni, D. (jibhervhis, Azygog. snecicus, and CnjiAog. 

 Ilojil'iitsoni. It is seldom well preserved; the best specimens known to us come 

 from the Skiddaw Slates, and are in the collections of the Woodwardian Museum, 

 the Keswick Museum, and in Postlethwaite's private collection. 



Tetragraptus crucifer (Hall). Plate V, fig. 2. 



1858. Oraptolitliits crucifer, Hall, Geol. Survey Canada Rep., 1857, p. 125. 



] 865. Graptolithiis crucifer. Hall, Grapt. of Quebec Group, p. 92, pi. v, fig. 10. 



1865. Tetragrapsus crucifer, Nicholson, Quart. Journ. Geol. Soc, vol. xxiv, p. 144. 



1898. Tetragraptus crucifer, Elles, ibid., vol. liv, p. 488, fig. 12. 



Main stipes long, about 8 cm. in length, approximately straight and rigid, but 



widening rapidly to a maximum width of 4 mm. Thecse about eight in 



10 mm., inclined at 45°, four times as long as wide, and having a length 



of about 3 — 4 mm. ; in contact for three (piarters tlieir extent. Apertural 



margins normal, concave. 



Description. — ^A suiall disc generally encloses both stipes of the first order, which 



ar(> short, and also the proximal ]:)ortions of the main stipes. In shape this disc, 



while i)i-ol)abIy oi-iginally square in form, is commonly oblong after compression, 



averaging 7 by 5 mm. ; it is almost invariably slightly prolonged up the stipes. 



Details of the plan of structure of the jjroximal end have not been observed, 

 and the thecal characters are also obscure. 



Ajfinities. — In general foi-m 7'. crucifer resembles both T. (/nadriltracJiiatiis and 

 T. Headi. As already ])ointcd out, it seems probable that the larger and wider 

 forms often referred lo 7'. quadritiracfiiatns would ])e more legitimately included in 



