42 BRITISH GRAPTOLITES. 



Didymograptus bifidus (Hall). Plate IV, figs. ] <i — f. 



1865. Grajifolithus bifidus, TTall, " G-rapt. of Quebec Group," Geol. Surv. Canada, Canadian Organic 



Remains, dec. 2, p. 73, pi. i, figs. 16 — 18 ; pi. iii, figs. 9, 10. 

 1868. Didymograpsns hificJus, Nicholson, Quart. Journ. Geol. Soc, vol. xxiv, p. 136. 

 1870. Didymograpsus hifidus, Nicholson, Ann. Mag. Nat. Hist. [4], vol. v, p. 346, fig. 7. 

 1875. Didymograptus hifidus, Hopkinson, Quart. Journ. Geol. Soc, vol. xxxi, p. 646, pi. xxxiii, 



figs. 8 a — c. 

 1898. Didymograjjtus bifidus, Elles, Quart. Journ. Geol. Soc, vol. liv, p. 511. 



Stipes from 1"3 to 4 cm. in length, very narrow at origin, but expanding 



continuously tlirougliout their length to a maximum width of 2"6 mm., 



and contracting rapidly at the distal extremities. Dorsal margin straight ; 



thecal margin curved. Sicula slender and tapering ; stipes diverging at 



varying angles, but always less than 90°. Tliec^e fifteen to thirteen in 10 mm., 



inclined at 45°, three to four times as long as wide, and free a quarter 



their length. Apertural margin normal, concave, mucronate. 



Description. — The stipes are very narrow at their origin, being about "8 mm. 



in width, but they ultimately attain a maximum l)readth of 2*(3 mm. Xone 



of our British specimens reach the dimensions of Hall's largest forms, either 



in length or width ; none are known from this country to exceed 5 cm. ; 



small forms about 2*5 cm. in length are most commonly met with. The 



greatest width observed hj Hall ((ro mm.) is never approached in any 



British specimen. The maximum width of the .stipes is attained near the distal 



end, after which there is a somewhat rapid diminution. The increase in width is 



due to the fact that the thecse augment steadily in length from the proximal to 



the distal portions of the stipes; but at the distal 

 Figs. 26 « aml^6^-^KJ,mo^r.p<»« extremities i\ie thecEe are only partially grown, and 



consequently diminution in width takes place. One 

 result of this mode of growth is to give a charac- 

 teristic curvature to the thecal margins ; these have 

 at first a slight concave curvature, but subsequently 

 the curve may be convex, or else continuously 

 concave. 



The sicula is long and narrow ; it has a length 

 -□.,,.„,,,.., of about 2'1 — 2-4 mm. The first theca (th. 1') 



«. Jfroximal end in lull relief, obverse ^ ' 



view. Pont Seiont. Coll. Wood- originates ou the left side of the sicula (obverse 



wardiau Museum. . '^ ^- ^ t 



b. Proximal end in full relief, reverse view) at some little distance above its aperture 



view, sbowins' crossing canal. / t , rj.ij_iTi^ j_j^i \ i 



Pont Seiont. (about One tourth the distance to the apex), and 



growing downward and slightly outward leaves the 

 sicula free on the left side. The crossing canal seen in the reverse aspect of the 

 polypary traverses the sicula almost horizontally at a short distance above its 



