Yol. 54.] FAUNA OP THE SKIDDAW SLATES. 503 



The stipes are usually rigid, and their dorsal walls straight, but 

 occasionally they may be slightly curved ; in some individuals they 

 are 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 commonly 

 rather 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 its 

 aperture. The earliest theca (th. 1^) arises about halfway up the 

 sicula on the left side (obverse view). The general structure of the 

 rhabdosoma is best seen in the reverse view. Theca 1^ at once 

 curves concavely outward, so that the sicula is free on the left side 

 from the point of origin of theca 1^ to its aperture. The connecting- 

 canal is long and oblique, and theca 1' is closely adpressed to the 

 right side of the sicula. 



The angle at which the stipes diverge from each other varies 

 from 110° to 130°. The outer walls of the earliest thecae have a 

 slight concave curvature, but in the thecae developed nearer the 

 distal end the walls are straight. The thecse are in contact with 

 each other for a short distance at their base ; they are narrow tubes 

 of uniform width, and are inclined to the general direction of the 

 stipe at an angle which is usually about 25°, but may vary from 

 20° to 30° in differerent parts of the stipe. 



The apertures of the thecse are concave ; they form with the axis 

 of the stipe an angle of 100° to 105°. The thecse are about 4 times 

 as long as they are wide. 



The species may be said to be characterized (1) by the rigidity 

 of the stipes ; (2) by the characters of the thecae : (a) the nearly 

 perpendicular aperture ; (b) contact only at base ; (c) low angle 

 of inclination; (3) by the number of thecae in a given unit of 

 length. 



Occurrence. — On one slab this species is associated with 

 Tetragraptus quadribrachiatus (Hall), Middle Skiddaw Slates. 



Localities. — Barf, near Keswick; Carlside Edge, Skiddaw; 

 Outerside ; Thornship Beck, Shap. 



(a) DiDTMOGEAPTFS AFFINIS, Xich. 



1863. Didymogra'psus sp., Salt. Quart. Journ. Geol. Soc. vol. xix, p. 137, fig. IZd. 



1869. Didymogra'psus affinis, Nich. Ann. Mag. Nat. Hist. ser. 4, vol. iv, p, 240 & 

 pi. xi, fig. 20. 



1870. Didymograpsus affinis, Nich. ihid. vol. v, p. 343, fig. 4. 



The specimens in the Woodwardian Museum which have been 

 referred to this species should, in my opinion, have been referred to 

 Didymograptus Nicholsoni, Lapw. ; they do not resemble Nichol- 

 son's types. 



In D. affinis the stipes rarely exceed J^- inch ('5 mm.) in width. 

 The cells are free for almost their whole length ; they number 

 eighteen to the inch, or perhaps twenty if the measurement be taken 

 near the sicula (seven to eight in 10 mm.) ; they are inclined at a 

 low angle, 15° to 20^, and their apertures are perpendicular to 

 the general direction of the stipe. 



