Vol. 54.] I'AUNA OF THE SKIDD AW SLATES. 497 



1891. ' Didymograptus' ^^66er•^t?t(s, Moberg, Geol. Foren. Stockholm Forhaudl. 

 vol. xiii, p. 221. 



1892. Isograptus gihheruUis, Moberg, ibid. vol. xiv, p. 346 & pi. viii, figs. 3-7. 

 1895. Didymograptus gibherulus, Holm, ihid. vol. xvii, p. 334. 



' E-habdosoma bilateral, consisting of two fairly broad stipes 

 which bend upwards from the sicula in gentle curves at an angle 

 of 335° to 340°. Stipes widest at their origin, attaining commonly 

 y2 inch (2'1 mm.) ; there is a marked distinction towards the distal 

 extremity, the stipe there seldom exceeding -^ inch (1"26 mm.) 

 in width. 



' The thecse number forty to the inch (sixteen in 10 mm.) ; the 

 apertures of the earliest cellules are directed obliquely downwards, 

 but the thecse then curve in such a manner that they come to face 

 horizontally outwards, and ultimately even upwards ; they are then 

 inclined to the general direction of the stipe at an angle of about 

 45°, and have an apertural angle of 130°. The thecse are in con- 

 tact for almost the whole of their length ; in the proximal part of 

 the rhabdosoma they are 4 times as long as wide, but in the distal 

 part of the stipes they are very much shorter.' 



This species is abundant in the Skiddaw Slates. There are 

 very good specimens in the "Woodwardian Museum, the Keswick 

 Museum, and in Mr. Postlethwaite's collection. Three of the speci- 

 mens in the "Woodwardian Museum are Nicholson's types. Another 

 is better preserved than any of these ; it is in relief, and exhibits 

 important structural details, to some of which reference has been 

 recently made by Dr. Holm in his work on the Swedish specimens. 



The species varies greatly in size and form, especially as regards 

 the direction of the stipes. These varieties may be distinguished as 

 follows : — 



(1) Dorsal vralls of stipes continuously curved, so that the distal extremities 



point tovrards each other ; 



(2) Dorsal walls of stipes straight ; stipes almost parallel to each other ; 



(3) Dorsal walls straight ; distal extremities of stipes directed away from 



each other. 



In aU these forms, however, details of structure are essentially 

 the same, and there appears to be every gradation between the 

 extreme types. 



The species bears a superficial resemblance to some aspects of 

 Tetragraptus Bigshyi (Hall), in which only two stipes are seen. It 

 should be readily distinguished from that species by the fact that in 

 T. Bigshyi the rhabdosoma is narrowest in its proximal part, 

 whereas in Didymograptus gihherulus it is widest at this point. 



Notes on Structure of Type-specimens and others. 



(A) Obverse Side.^ 



1. — Impression of obverse (sicula-side) aspect of a specimen of 



' For an explanation of the terms obverse aspect and reverse aspect, 

 see my paper ' On the Subgenera Petalograjptus & Cephalograptus,' Quart. 

 Journ. Geol. Soc. vol. liii (1897) p. 189. 



2m2 



