DIDYMOGRAPTUS. 



29 



Didymograptus Nicholsoni, var. planus, Nov. Plate II, figs. 5 a, J). 



Figs. 17 a ami b. — Didymograptus 

 Nicholsoni, var. iilanus, nov. 



In addition to the typical form of D. Nicholsoni there also occurs a variety 

 differing from the type in general shape, and in the lower inclination of the 

 thecae, though agreeing with it in all other essential particulars. The stipes 

 run horizontally, except in the neighbourhood of the sicula, where they 

 exhibit marked convex curvature. The inclination of the thecas seems never 

 to lie as high as 25°, but always exceeds 20°. 



In the true D. NieJiolsoni the angle of divergence 

 never exceeds 150°, and the inclination of the thecae 

 in the distal parts of the stipes is almost invariably 

 as high as 30°, and these characters are practically 

 constant in the many specimens which we have 

 examined from different parts of the country. 



Horizon ami Lordlif.ips. — D. Nicholsoni, var. 

 plamis, occurs in the Upper Skiddaw Slates. 

 Lake District : Outerside. 



Specimens of this variety, including our type 

 specimen (PI. II, fig. 5 a), arc in the Woodwardian 

 Museum. 



a. Proximal end, obverse view. En- 

 largement of part of PI. II, fig. 5 a. 



h. Distal thecfe. Enlargement of 

 part of Pi. 11, fig. 5 b. 



Didymograptus serratulus (Hall). Plate II, figs. 7 a, h. 



1847. GraptoUthus serratulus, Hall, Pal. N. York, vol. i, p. 274, pi. Ixxiv, fig. 5. 

 1868. Non Didymorjrapsus serratulus, Nicholson, Quart. Journ. Geol. See, vol. xxiv, p. 136. 

 1870. Non Didipmxjrapsus serratulus, Nicholson, Ann. Mag. Nat. Hist. [4], vol. v, p. 343, pi. vii 

 figs. 3, 3 (I. 



Stipes 5 — 7 cm. in length (or even more), uniformly narrow, not exceeding 



1 mm. in width, straight or slightly curved, diverging from a conspicuous 



sicula at about 120°. Thecae seven to eight in 10 mm., three to four times 



as long as wide, overlap increasing with age of stipe, inclined at an average 



angle of 25°. Apertural margin straight, practically normal. 



Bcsci'iption. — The stipes vary from "8 to 1 mm. in width, being practically 



uniform throughout. The only specimen known to us which exhibits any details 



of structure of the proximal end is seen in the reverse aspect. The sicula is 



2 mm. or more in length, and its aperture almost entirely concealed by the second 



theca. The origin of the first theca is obscure, but it is almost certainly suboral. 



The apical end of sicula is prolonged into a fine thread {nemo), some 6 mm. in 



length, which appears to branch at its extremity. 



