DIDYMOGRAPTUS. 



27 



Didymograptus Nicholsoni, Lapworth. 



Plate II, figs. 4 a- 



FiGS. 16 a and h. — Didymograptus Nichohoid, 

 Lapwortli. 



1868. Didymograpsus serratulas, Nicholson, Quart. Jouru. Geol. Soc, vol. xxiv. p. 136. 



1870. Dldyniograpsus serratulus, Nicholson, Ann. Mag. Nat. Hist. [4], vol. v, p. 343, pi. vii, figs. 3, 3 d. 



1875. DidymograptiiK Nicholsoni, Lapworth, Quart. Jovuni. Geol. Soc, vol. xxxi, p. 644, pi. xxxiii, 



figs. 5 a — d. 

 1898. Didymograptus Nicholsoni, Elles, Quart. Journ. Geol. Soc, vol. liv, p. 502, fig. 21. 



Stipes from 2*5 to 5 cm. in lengtli, uniformly slender, never exceeding 1 mm. in 



width, diverging from the sicula at angles of 110° — 130°. Thecae ten in 



10 mm., four to five times as long as wide, in contact for a small fraction of 



their length, inclined at 25° — 30°, Apertural margin normal, straight. 



Description. — The narrow stipes are usually rigid and their dorsal walls straight, 



but occasionally they may be slightly curved ; in some individuals they attain a 



great length (11 cm.), but smaller specimens (4 cm.) are commoner. They widen 



very slightly from their point of origin, l)ut are uniformly narrow for the greater 



part of their length, never exceeding 1 nnn. in width, and measuring commonly 



rather less ("75 mm.). 



The sicula is about 1*8 mm. in length; it 

 is narrow, and does not widen much, even in 

 the direction of its aperture, where it mea- 

 sures '3 to "4 mm. The earliest tlieca (th. 1') 

 originates about halfway up the sicula on 

 the left side (obverse ^'iew). The general 

 structure of the polypary is, however, best 

 seen in the reverse view. Theca 1^ at once 

 curves outward, leaving the sicula free on 

 the left side near its aperture. The crossing 

 canal is long and oblique, and theca V is 

 closely appressed to the right side of the 

 sicula. Of the thecee developed subsequently, 

 those nearest the sicula show a slight concave curvature of their outer walls, 

 but the walls of the more distal thecse are straighter. 



The number of tlieca3 in a given unit of 

 leng-th is also somewhat variable ; there are 

 usually ten in 10 mm., but they are commonly 

 more remote in the initial parts of the stipes. 

 The average inclination to the axis is 25° to 

 30°, but varies from 20° to 30° in different 

 parts of the stipes. In the distal parts the 

 thecae are in contact for half to one third their length, but near the sicula the over- 

 lap is less. The average thecal length is about 1*6 nun. 



a. Proximal end, cast, reverse view. Barf. 



Coll. Woodwardian Museum. 



b. Proximal end, obverse view. Enlarge- 



ment of part of PI. II, fig. 4 a. 



Fig. 16 c- 



-Didymogruptus Nicholsoni, 

 Lapworth. 



Distal theca;, cast. Enlargement of part 

 of PI. II, fisr. 4 a. 



