DIDYIIOGRAPTUS. 37 



the Skidd aw Slates only, in which it is fairly abnndant ; good specimens are in 

 the collection of the Woodwardian Mnsenm and of the Geological Survey, and in 

 that of ]Mr. John Postlethwaite, of Keswick. Its associates are unknown. The 

 type specimen (PL II, fig. 12 a) is in the Woodwardian Museum, 



Dependent Series. 



Didymograpti with stipes diverging downward, and in which the stipes tend to 

 become approximately parallel. 



Group V. — Type D. Murchisoni. 



Didymograpti in which the stipes grow downward; they originate slightly 

 above the aperture of the sicula ; the thecse are inclined at large angles, and the 

 amount of overlap is considerable. 



Sub-group A. — Type D. hifidiis. 

 Dependent Didymograpti in which the stipes widen throughout their length. 



Didymograptus Murchisoni (Beck). Plate III, figs. 1 a — /i-. 



1839. Graptolithus Murchisoni, Beck, Murchison's Sil. System, pi. xxvi, fig. 4. 



1861. Diclymograpsus Murchisoni, Baily, Journ. Geol. Soc, Dubliu, vol. ix, pi. iv, figs. 1 a — c. 



1869. Didymograpsus Murchisoni, Hopkinson, Journ. Quekett Micro. Club, vol. i, pi. viii, figs. 6 a, b. 



1870. Didymograpsus Mui'chisoni, Nicholson, Ann. Mag. Nat. Hist. [4], vol. v, p. 349, pi. vii, figs. 

 7, 7 a, 7 b. 



1875. Didymograptus Murchisoni, Lapworth, Quart. Journ. Geol. Soc, vol. xxxi, p. 648, pi. xxxv, 

 figs. 2 a — f. 



Stipes robust, from 5 to 7"5 cm. in length, somewhat narrow at their 



commencement, but expanding gradually in width to a maximum diameter 



of 8 to 4 mm., and then continuing without further widening ; diverging from 



a large blunt sicula at a primary angle of about 40°, curved at their origin 



but almost immediately becoming straight and parallel. Thecas twelve to 



fourteen in 10 mm., inclined to the axis at an angle of 45"^, three to four 



times as long as wide, and free one half to one fifth their length. 



Apertural margin normal, concave, with an acute denticle. 



Description. — The gradual widening of the stipes from tlieir origin is eminently 



characteristic of the species ; in the typical forms the stipes are parallel after the 



development of the first dozen thecae, and widen but little. In some of the longer 



