The Quarterly journal of the Geological Society of London . uart. Jouvn. Geol. xxiv, p. 136. 1870. Didymograpsxis serraUdus, Nich. Ami, Mag. Nat. Hist. ser. 4, vol. v, p. 343& pi. vii, figs. 3-3 d. 1875. Didymograjptus Nicliolsoni, Lapw. & Hopk. Quart. Soc. vol. xxxi,p. 644 & pi. xxxiii, figs. 5 a-d. The Skiddaw Slate specimens now referred to D. Nicliolsoniwere provisionally re-ferred by Nicholson Fig. 21.—Impression of Didymograptusto D. serratulus. As Nicholsoni from Barf near Keswick; Lapworth has shown, coll. Woodivardian Museum. Halls species differsfrom this in the
The Quarterly journal of the Geological Society of London . uart. Jouvn. Geol. xxiv, p. 136. 1870. Didymograpsxis serraUdus, Nich. Ami, Mag. Nat. Hist. ser. 4, vol. v, p. 343& pi. vii, figs. 3-3 d. 1875. Didymograjptus Nicliolsoni, Lapw. & Hopk. Quart. Soc. vol. xxxi,p. 644 & pi. xxxiii, figs. 5 a-d. The Skiddaw Slate specimens now referred to D. Nicliolsoniwere provisionally re-ferred by Nicholson Fig. 21.—Impression of Didymograptusto D. serratulus. As Nicholsoni from Barf near Keswick; Lapworth has shown, coll. Woodivardian Museum. Halls species differsfrom this in the numberof thecae in a givenunit of length and intheir shape. Accordingto Halls figure, theremust be only sixteenthecse in the spaceof 1 inch (six in10 mm.) in D. serra-tulus, whereas in there aretwenty - six (rathermore than ten in 10mm.). The original speci-mens obtained fromthe Skiddaw Slatesare said to have beenimperfect, but somespecimens of the formcollected from Barfand now in the Wood-wardian Museum arevery well [X 1.^] Yol. 54.] FAUNA OP THE SKIDDAW SLATES. 503 The stipes are usually rigid, and their dorsal walls straight, butoccasionally they may be slightly curved; in some individuals theyare very long. The stipes widen slightly from their point of origin,but they are uniformly narrow for almost their entire length,never exceeding J^ inch (1-05 mm.) and measuring commonlyrather less. Notes on Structure.—The sicula is y^^r inch (1*58 mm.) long ;it is narrow, and does not widen much even in the direction of itsaperture. The earliest theca (th. 1^) arises about halfway up thesicula on the left side (obverse view). The general structure of therhabdosoma is best seen in the reverse view. Theca 1^ at oncecurves concavely outward, so that the sicula is free on the left sidefrom the point of origin of theca 1^ to its aperture. The connecting-canal is long and oblique, and theca 1 is closely adpressed to theright side of the sicula. The angle at whi
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