BRITISH GRAPTOLITES. 



Horizontal Series. 



Didymograpti with approximately straight and horizontal stipes in -wliicli the 

 ultimate angle of divergence is 180°. 



Group I. — Type D. exten.ms. 



Didymograpti in Avliicli the stipes originate near the apex of the sicula and 

 grow horizontally. Their thecae are inclined at large angles, and overlap con- 

 siderably. 



Didymograptus extensus (Hall). Plate I, figs, la, h. 



1858. Graptolithiis exte7isvs, Hall, Rep. Geol. SurTey Canada, 1857, p. 132. 



1865. Graptolithvs exfensns. Hall, " Grapt. of Quebec Group," Geol. Surv. Canada, Canadian Organic. 



Remains, dec. 2, p. 80, pi. ii, figs. 11 — 16. 

 1870. Buhjmociraptvs extensus, Nicholson, Ann. Mag. Nat. Hist. [4], vol. v, p. 341, pi. vii, figs. 2, 2 a. 

 1875. Didi/mograjdvs extensus, Hopkinson, Quart. Jouru. Geol. Soc, vol. xxxi, p. 642, pi. xxx, figs. 



1 a — d. 

 1898. Didymograptus extenaus, EUes, Quart. Journ. Geol. Soc , vol. liv, p. 504. 

 1901. Didymograptus extensus, Tornquist, Lunds Univ. Arrsk., Bd. xxxvii, Af. 2, Nr. 5, p. 14, pi. i, 



figs. 25—30. 



Stipes diverging at 180°, 20 — 30 cm. in length, narrow, widening slightly and 

 gradually from their origin throughout their length to a maximum of 

 1'6 mm. Thecfe nine to ten in 10 mm., inclined at a fairly large angle, 

 three times as long as wide, and free one third their length. Apertural 

 margins normal, concave. 

 Descripfio)! . — The stipes are characteristically slender and flexuous. They widen 

 very gradually for the first 12 mm. of their length, and still more gradually after- 

 wards. The width at origin is generally about "5 mm. ; this may increase to a 

 maximum of 1*(3 mm., but more commonly the stipes do not exceed 1 mm. in breadth. 

 They seem occasionally to have attained a great length, one specimen known to us 

 measuring fully 30"5 cm. from end to end. In such long forms there is always a 

 distinct tendency to upward curvature of the stipe in the distal region. There is 

 also a further curvature of the celluliferous margins accompanied by a diminution 

 in width ; this is due, as in other cases, to the incomplete development of the latest 

 formed thecae. 



The sicula is small and inconspicuous ; it never exceeds 1"3 mm. in length. 

 The earliest theca (th. 1^) originates in the apical region of the sicula, and grows 

 in such a manner that the sicula is free on the left side for a short distance near 

 its aperture. It is possible that th. 1" also originates near the apex of the 

 sicula, since the appearance of the polypary in the reverse aspect suggests the almost 



