DIDITMOGRAPTUS. 51 



and remarkably narroAV width, inclined at 1-5°. Apertnral margins concave, 



and somewhat obliqne. 



Descnption. — Examples of this species are found, as a rule, in a fragmentary 



condition ; those showing the proximal end are rare. The dorsal wall of each stipe 



is concavely curved, but the amount of curvature varies in different examples. Each 



stipe starts outwards horizontally, and then curves dorsally. In some specimens the 



curvature becomes almost that of a semicircle, so that the distal portion of the 



stipe is parallel with the proximal part ; but in others, after the first decided 



concave curve, the stipe runs very nearly straight in a horizontal direction. 



Figs. 32 a and b.-Didymograptusfas- The thecae are peculiar and eminently character- 



ciculahis, Nicholson. j^tic. In the distal parts of the stipes they number 



nine to ten in 10 mm., and are in contact for foui- 

 fifths of their length ; nearer the proximal end the 

 thecae are more distant, are always smaller, and are 

 in contact with each other for a very small fraction 

 of their length. The thecae are long, narrow-linear 

 tubes, and may attain a length of 6'3 mm. ; their 

 width is so slight that a line drawn perpendicular 

 to the axis of the stipe and near the aperture of 

 « h the theca will cut four different thecae, and yet 



<■( Proximal thecae, showing com- -, •iici • -it i i 



parativeiy small amount of over- the Width ot the stipe, mcluduig tliesc thccae, uever 



PL II, fig'sT"'""* °* ^^'*' ""^ exceeds M mm. In their form and especially in the 



6. More distal thecae, showing large characters of their aperturcs the thec« shoAv an 



amount or overlap and oblique '^ 



apertures. Enlargement of part approach to the Lcptograptid type. 



of PI. II, fig. 8 «. T 1 T 1 • Cl- 



in the distal portions ot the stipes the great 



length of the thecae, their concave curvature, and small angle of inclination make 



them appear to run nearly parallel to the back of the stipe. 



Affinitieii. — The peculiar characters of the thecae and the curvature of the 

 stipes readily distinguish the species from all others at present known, and render 

 its determination easy even when it is in a fragmentary condition. 



Asi^ociatps, etc. — Up to the present time D. fasriculatas has only been recorded 

 from the Upper Skiddaw Slates, and it is a rare fossil in those beds. It occurs asso- 

 ciated with rJiyllograptus, cfr. tiipHf^. The original specimen figured by Nicholson 

 shows two stipes. This we have been unable to discover, but we would provisionally 

 regard the specimen figured on our PL II, fig. 8 a, as constituting the type of 

 this species. A few specimens labelled by Nicholson are in the British Museum ; 

 others, originally in his private collection, have been presented to Lapworth by Dr. 

 H. 0. Nicholson ; while a few indifferent specimens are in the Woodwardian 

 Museum, Cambridge. 



Hovizoii and Localities. — Arenig, Upper SkiddaAv Slates (Ellergill beds). 



Lab' District : Aik Beck, Pooley ; Thornship Beck ; Ellergill. 



