CLIMACOGRAPTUS. L95 



included between them appears to vary from (i() n to 140°. They arc frequently 

 enclosed partially or entirely within a membrane, which gives a false impression 

 of their actual width. 



The appearance of the thecee varies very much with preservation. They are 

 comparatively rarely presented in true profile view; when so presented, however, 

 while a marked sigmoid curve is characteristic of the line of the ventral wall, the 

 free edge is practically straight. Each theca starts from the inner part of the 

 theca lying next below, and almost at once grows outward and upward, so thai 

 the greater part of its ventral wall is free ; the amount of overlap is approximately 

 one-third, or rather less. The theca? are, however, often preserved in sub- 

 scalariform view, when the aperture is seen to be elliptical in outline. In 

 profile view the thecal apertures are markedly alternate in their arrangement, 

 particularly so at the proximal end. The septum is complete on the obverse side, 

 but possibly incomplete at the extreme proximal end on the reverse side. 



Affinities. — The presence of the basal spines, the characters of the theca?, and 

 the tapering of the polypary in a proximal direction are characteristics which 

 serve to separate Gl. bicornis from other Climacograpti. 



Horizon and Localities. — Upper Glenkiln and Hartfell Shales. 



S. Scotland : Dobb's Linn ; Hartfell ; Kirkhill, Wanlock Head ; Berrybush 

 Burn ; Black Linn ; Glenkiln Burn ; Craighead ; Laggan Gill ; S. of Ardwell ; 

 Penwhapple Glen, etc. Wales: Tiddyndicwm; Derwendeg; TyFry; Prendergast 

 Farm, Haverfordwest; Railway Pendwr, S. Bletherston ; near Builth, etc. 

 Ireland: Tinnaclough, Co. Wexford. 



Associates, etc. — Gl. bicornis is an exceedingly abundant fossil in the Glenkiln 

 and Lower Hartfell Shales. In the Glenkiln Shales it occurs in the Nemag. 

 gracilis zone associated with N. gracilis, Dicellog. sextans, and Glimacog. Scharenbergi; 

 in the immediately overlying zone of Dicellog. patulosus it occurs with Gl. peltifer 

 and the zone fossil; while in the Lower Hartfell Shales it abounds in the 

 Gl. Wilsoni zone, associated with Gl. Wilsoni, Gl. Scharenbergi, and l)i<-r<in<>g. 

 Nicholsoni. Some individuals are met with in the higher zone of Dicranog. 

 Glingani, associated with D. Glingani, Pier/lug. Morrisi, Dicranog. ramosus, and 

 Leptog. flaccidus. 



Collections. — Geological Survey of Scotland, Sedgwick Museum, Lapworth, and 

 Authors. 



Var. tridentatus, Lapworth. Plate XXVT, figs. 9 a — c. 



1870. Climacograptus bicornis, var. tridentatus, Lapworth, Cat. West Scot. Fosr., pi. ii, li--. 52. 

 1877. Climacograptus bicornis, var. tridentatus, Lapworth, Grap. Co. Down, p. 139, pi. vi, fig. 38 c. 



There is a well-marked variety of Gl. bicornis in which the virgella is so 

 developed that it constitutes a third long basal spine; it is often fully 2mm. in 



