oOS BRITISH GRAPTOLITES. 



appearances which they present. There are commonly eight to five thecge in the 

 interval separating the different cladia, hut there is no absolute regularity in this 

 respect. 



Affinities. — The only other Cyrtograptus which approximates to a sub-circular 

 form is Cyrtog. Marciiisoni, and this differs from Cyvtog. Luvdgreni in the jjosses- 

 sion of a greater number of cladia, and in the character of the thecse. 



Horizon and TjocaUties. —Wenlock Shales (zone of Cyrtog. Lnndgreni). 



Welsh Border: Ackley Lane, near Chirbury; Lhvynrhedith Quarrj^, Ackley 

 Lane, near Chirbury ; Sale Brook, near Middletown ; Trewern Brook, near Middle- 

 town ; River Irfon, Builth. Lahe District : Cautley, near Sedbergh. 



Associates, etc. — Cyrtog. Lundgreni is often quite abundant in the highest beds 

 of the Wenlock Shales, where it occurs with Monog. Flemiiigi, and var. compactus, 

 M. duhiuH, and Cyrtog. Car rathe rsi. 



Collections. — Watney and Welch, Mr. Macgregor of Queen's College, Cambridge, 

 and the Authors. 



Cyrtograptus rigidus, Tullberg. Plate LII, figs. 2 a — c. 



1883. Cyrtograptus rigidus, Tullberg, Sk&nes Grraptoliter, ii, p. 38, pi. iv, figs. 12 — 14. 

 1900. Cyrtograptus rigidus, Elles, Quart. Journ. Greol. Soc, vol. Ivi, p. 409, pi. xxiv, figs. 2, a, 

 B, c, ai]d text-fig. 23. 



Polypary with a stipe several centimetres in length, and possessing a single 

 cladium only ; stipe, with long, slender, gracefully curved proximal 

 portion, widening gradually and persistently from a small inconspicuous 

 sicula till the maximum breadth of 1'5 mm. is attained. Theca3 ten to eight 

 in 10 mm.; those of the proximal portion sub-triangular with reflexed 

 apertural margin, in contact only, but becoming more tubular and over- 

 lapping fully one third to one half their length in the proximal portion 

 where the number of thecae never exceeds eight in 10 mm. ; characters 

 of the cladium in all respects like those of the distal portion of the stipe. 

 Description. — When well preserved the polypary often shows graceful curva- 

 ture of the stipe, and the cladium is the exact counterpart of the termination of 

 the stipe, the curvature being equal in amount, but opposite in direction. There 

 are in all about eighteen tliecee of the proximal type, and those in the region of 

 the inconspicuous sicula are very small, the total breadth of the stipe in this 

 initial portion never exceeding '5 mm. 



The change in the form of the thecse is chiefly brought about by elongation 

 relatively to breadth and by increased amount of overlap. Of the eighteen thecas 

 of the proximal type thirteen are usually developed before the cladium is given 

 off. The proximal end is straight and never enrolled, though from the frequency 



