DIDYMOGRAPTUS. 



31 



Figs. 19 a and b. — Didyinograptus 

 simulans. sp. iiov. 



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a. Proximal end, obverse view. En- 



largement of part of PI. 11, 

 fig. 6 b. 



b. Proximal end, in low relief, reverse 



view. Barf. Coll. Woodwardian 

 Museum. 



Description. — The dorsal wall of eacli stipe shows slight convex curva- 

 ture in its initial region, but it becomes straighter distally. The increase in 

 width is fairly rapid at first, but is more gradual in the distal parts of 

 the stipe ; the width at origin is aV)out •() mm., and the maximum observed 

 1"4 mm. 



The sicula is slender and tapering, and measures 

 1*.54 mm. in length. Theca 1' originates some little 

 distance above the base, and appears at once to grow 

 outward and downward with a graceful doul)le 

 curvature of its outer wall. The crossing canal is 

 narrow. It crosses the sicida obli(|uely a little 

 above the sicula aperture, to which theca 1** is 

 closely appressed. This gives the sicula the 

 appearance of being obliquely situated with respect 

 to the two stipes. 



The thecae, which have an average length of 

 2 mm., are simple, and widen slightly in the direc- 

 tion of their apertures. The overlap is hardly 

 appreciable in the initial part of the stipes, but the 

 overlap increases to one half the length in the distal region. The apertural 

 angle is commonly about 120° — 125°, but occasionally the thecse have a " coii- 

 utrictiform " appearance. 



Afflidfu'.i. — D. .si in II I a IIS is allied to D. Nicliolsoid 

 and D. nifidus. It resembles D. NiclioUoni in the 

 character of its proximal end, which is almost 

 identical with the ])i'oximal end of that species, 

 but is distinguished by its more closely set thecae. 

 In the individual characters of the thecae, on^ the 

 other hand, this species approaches D. nitidus, but 

 the sicula is different in form, and the whole 

 character of the proximal end distinct. 



]J. simidans was originally referred by Nicholson to D. nifidMS of Hall 

 (loc. cit.), but is easily distinguished from that species by the characteristics 

 above mentioned. 



Horizon and Localities. — Arenig (Middle Skiddaw Slates). 

 Lalce District : Barf ; Randal Crag. 



Associates, etc. — D. mnidans appears to be confined to the Skiddaw Slates, 

 in which it occurs in great numbers at certain horizons. It has not yet been 

 found associated with any other species. Specimens collected from the Lake 

 district, including the type specimen (PI. II, fig. 6 />), are in the Woodwardian 

 Aluseum. 



Fig. 19 c. — Didymograptus simulans, 

 sp. nov. 



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c. Distal thecae, preserved as a cast, 

 showing growth-lines. Enlarge- 

 ment of part of PI. II, fig. <) b. 



