r,2 BRITISH GRAPTOLITES. 



Reclined Series. 



Didymograpti in which the stipes are approximately straight and are directed 

 upward and backward towards the nema, the initial and ultimate angles of diver- 

 gence being both more than 180°. 



Group VII. — Type D. gibberulus. 



Didymograpti in which the stipes slope upward ; they originate from near the 

 a])ex of the sicula ; the thecse are numerous, and are in contact throughout their 

 length. 



Didymograptus gibberulus, Nicholson (= gen. Isograptus, Moberg). Plate II, 



fig'S. 9 a — e.' 



^c^^ 



1853. Bidymograpsus cadvceus, Salter (pars), Quart. Journ. Geol. Soc, vol. ix, p. 87, fig. 1 a. 

 1863. Bidymograpsus cadiiceus, Salter (pars). Quart. Journ. Geol. Soc, vol. xix, p. 138, fig. 13 a. 

 1875. Bid ymograptns gibberulus, Nicholson, Ann. Mag. Nat. Hist. [4], vol. xvi, p. 271, pi. vii, figs. 



3, 3 a, b. 

 1875. PlnjUograptvs stella, Hopkinson, Quart. Journ. Geol. Soc, vol. xxxi, p. 658, pi. xxxiv, fig. 6. 



1891. Bidyinograptus gibberulus, Moberg, Geol. Foren. Stockholm Forhiindl., vol. xiii, p. 221. 



1892. Isograptus gibberulus, Moberg, Geol. Fiireu. Stockholm Forhiindl., vol. xiv, p. 346, pi. viii, 

 figs. 3 — 7. 



1895. Bidymograptus gibberulus. Holm, Geol. Foren. Stockholm Forhiindl., vol. xvii, p. 334. 

 1901. Isograptus gibberulus, Tiirnquist, Luuds Univ. Arsskr., Bd. xxxvii, Ai. 2, No. 5, p. 23, pi. iii, 

 figs. 16-19. 



Stipes short, decreasing in width from the common point of origin, where they 



are 2"1 mm. wide, and forming a polypary of a horseshoe shape. Sicula 



long and slender, identical in form with and s^^mmetrical in position with 



respect to the first theca. Thecse sixteen in K) mm., curved, inclined to 



axis at 45°, about four times as long as wide, in contact throughout their 



length. Apertural margin concave, angle obtuse. 



DcHcripHon. — The stipes are widest at theii- origin, attaining commonly a 



breadth of 2'1 nmi. ; there is marked diminution towards the distal extremities, 



where the width seldom exceeds 1*5 mm. The ultimate form of the polypary 



' The specimens originally figured by Salter as Bidymograpsus cadiiceus included forms belonging 

 to two different genera, Didymograptus and Tetragraptus. The Tetragraptus seems to be identical 

 with T. serra (Brogu.), and tlie Didymograptus agrees exactly with the form described later by 

 Nicholson as B. gibberulus. This being the case we decide to retain Nicholson's specific name rather 

 than the earlier name of Salter, in order to avoid the possibility of any ambiguity. 



