68 BRTTTSH GRAPTOLITES. 



CTiroup V. — T_ypc T. ]h'(jf^h//i. 



Tetragrapti in wliich tlic stipes are flexed and are directed outward or curve 

 backward and inward ; they are commonly short and wide ; tlie sicnla is con- 

 spicuous ; the thecse are inclined at high angles, and are in contact for the greater 

 part of their length. 



Tetragraptus Bigsbyi (Hall). Plate VI, figs. 6 a—e. 



1858. Phyllograptus similis, Hall, Geol. Survey Canada Eep., 1857, p. 140. 



1865. Graptolithus Bicfshyi, Hall, Grapt. of Quebec Group, p. 86, \A. xvi, figs. 22 — 30. 



1895. Tetragraptus Bigsbyi, Holm, Gcol. Foren. Forliandl., vol. xvii, p. 319. 



1898. Tetragraptus Bigsbyi, Elles, Quart. Journ. Geol. Soc, vol. liv, p. 489. 



Main stipes short and wide, typically curved, resulting in a joolypary of varied 



form, narrowest at their origin, but widening quickly to their maximum width 



of about 3'2 mm. Sicula long and conspicuous. Thecoe fourteen to thirteen 



in 10 mm., inclined at high angles, four times as long as wide, in contact for 



almost their entire length. Apertural margins slightly concave, and with 



mucronate denticles. 



Description. — Perhaps the commonest form of the polypary, as shown when 



flattened upon the rock, is best described as a broad oval, truncated at its upper 



end in most specimens, but occasionally nearly complete. There is a decided 



tendency on the part of some stipes to come into contact at their distal extremities, 



and in a few examples actual adhesion, and possibly fusion, appear to have 



occurred (PI. VI, figs. 6 d, c). Examples, however, with stipes which diverge 



continuously, are very common. The phenomenon of possible fusion is of great 



interest when regarded in connection with Phyllograptus (see aiifc, p. 56), but it 



cannot as yet be regarded as proved in this species. Should it be possible to do 



this at any future time, it might be advisable to separate forms possessing this 



characteristic, as Marr and Nicholson have suggested (' Geol. Mag.,' dec. 4, vol. ii, 



p. 538), under the name of T. inosculans. 



The curved stipes in T. Bigsbyi are usually short; they never exceed 19 mm. 

 in length, and forms with stipes of about 12"5 mm. are more common. At their 

 origin the stipes are only about 1 mm. wide, and the increase up to 3-2 mm. takes 

 place rapidly. They are wide in proportion to their length, and their ventral 

 margins arc always convex, though the rate of curvature varies in different 

 individuals. 



