TETRAGRAPTUS. 57 



Tetragraptus quadribrachiatus (Hall). Plate V, figs. 1 n—d. 



1858. Grapfolithus cpfadribrachiatu.% Hall, Geol. Survey Canada Rep., 1857, p. 125. 



1863. Tetragrapsus crucialis, Salter, Quart. Journ. Geol. Soc, vol. xix, p. 137, fig. 8 6. 



1865. Grapfolithus quadribrachiatus, Hall, Grapt. of Quebec Group, p. 91, pi. v, figs. 1 — 5, pi. vi, 



figs. 5, 6. 

 1868. Tetracjyapsiis qitadribrachiains, Nicholson, Quart. Journ. Geol. Soc, vol. xxiv, p. 131. 



1874. Tetragraptus quadribrachiatus, Etheridge, jun., Ann. Mag. Nat. Hist., ser. 4, vol. xiv, p. 1. 



1875. Tetragrapitus quadribrachiatus, Hopkinson and Lapworth, Quart. Journ. Geol. Soc, vol. xxxi, 

 p. 649, pi. xxxiii, figs. 9 a, b. 



1877. Graptolithus {Bidymograpsus) quadribrachiatus, M'Coy, Prodronius Pal. Victoria, pt. 1. 

 1898. Tetragrapitus quadribrachiatus, Elles, Quart. Journ. Geol. Soc, vol. liv, p. 485. 



Main stipes 2'5 — 7*5 cm. in length, approximately straight and rigid, somewhat 



slender at origin, bnt increasing gradually up to a maximum width of 



2*6 mm. Theca? ten to nine in 10 mm., inclined 30° to 40'^, four times as 



long as wide, free one half to one third their length. Apcrtural margins 



normal, straight, or with slight curvature. 



Description. — The stipes of the first order are short, and occasionally enclosed 



in a disc, though this is often absent. The main stipes may attain a length of 



7"5 cm. They increase in width from al)out 'G mm. at their origin to a maximum 



of 2*0 mm. Smaller examples of the species whose width is P2 — P5 mm. are 



of commoner occurrence. 



All details concerning the structure of the proximal end are obscure in our 

 British specimens, but it appears certain that the dichotomous division of the 

 stipes of the first order does not take place until at least two thectfi have been 

 already developed on each side of the sicula. The apertural part of the sicula 

 is all that is usually seen. 



Each theca measures about 1*6 mm. in length, 



Pig. 34. — Tetragraptus qtiadribracM- ,, ^ n • t i j.i j i i 



atus. Hall. the outcr wall IS slightly curved, and, as a general 



^;- vp ;: -v ^^ ^-^.y ■■>■ ^ V ■ rule, each theca is free for about half its length near 



"^^^^"^^V t,l^e proximal end of the polypary, but rather less 



Distal thccK. Enlargement of part of distally, and in large specimens for not more than 



one third its length. It is, however, often very 

 difficult to determine any of the characters of the thecae in the proximal parts of 

 the main stipes, since they are generally preserved with their apertures embedded 

 in the rock, and thus only the dorsal walls of the stipes are presented to the 

 observer. The apertural angle, though typically 90°, varies with compression 

 from 95° to 100^ 



