TETRAGRArTUS. 65 



Associates, etc. — Tlic associates of T. rostlctliindtij are at prosoiit iiiiknowii. 

 All the specimens known at present come from tlie Skiddaw Slates, and witli one 

 exception are in Postlethwaite's collection. 



Reclined Series. 



Tetragrapti Avitli two pairs of approximately straight stipes converging upward 

 and backward upon the nema, and making with it an angle of more than 180°. 



Group IV. — Type T. serra. 



Tetragrapti in which the main stipes are straight and directed obHquely upw^ard ; 

 they are commonly short. The sicula is conspicuous. The thecas are inclined at 

 high angles, and are in contact for the greater part of their length. 



Tetragraptus serra (Brong.). Plate VI, figs. I a^^f. 



1828. Fucoides serra, Brougniart, Hist. Veget. Foss., vol. i, p. 71, pi. vi, figs. 7, 8. 



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



1858. GraptoUthus hryonoides. Hall, Geol. Survey Canada Eep., 1857, p. 12G. 



1863. Tetragrapsus [bryo?ioides'], Saltei', Quart. Journ. Geol. Soc., vol. xix, p. 137, fig. 8^. 



1863. Didymograpsus caduceus, Salter, ibid., fig. 13 h. 



1865. Graptolithus hryonoides. Hall, Grapt. of Quebec Group, p. 84, pi. iv, figs. 1 — 8, 11. 



1868. Tetragrapsus hryonoides, Nicholson, Quart. Journ. Geol. Soc, vol. xxiv, p. 131. 



1868. Didymograptus caduceus, Nicholson, ibid., p. 133. 



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



1875. Tetragraptus serra, Hopkinson and Lapworth, Quart. Journ. Geol. Soc, vol. xxxi, p. 650, 

 pi. xxxiii, fig. 10. 



1875. Tetragraptus Halli, Hopkinson, ibid., figs. 11a, 11 h. 



1877. Graptolithus (Didymograpsus) hryonoides, M'Coy, Prodronms Pal. Victoria, dec. 1. 



1898. Tetragraptus serra, Elles, Quart. Journ. Geol. Soc, vol. liv, p. 490. 



Main stipes 2*5 — 5 cm. in length, approximately straight, narrow at their origin, 



but widening abruptly and rapidly to a maximum breadth of 3-2 mm. 



Sicula conspicuoiis ; thecee eight to nine in 10 mm., inclined at 45°, three to 



four times as long as wide, free one third to one fourth their length. 



Apertural margins normal, concave. 



Description. — The stipes of the first order attain together a length of about 



2 mm. The main stipes resulting from the dichotomous division of these do not, 



as a general rule, exceed 5 cm. in length. These stipes are quite narrow at their 



