CLIMACOG RAPTUS. 199 



In 01. Wilsoni the thecae number twelve to eight in 10 mm., they have an 

 average length of 2 mm., and overlap one-third to one-half their extent; the free 

 part of the outer wall tends to eurve slightly inward; the apertural margins are 

 approximately horizontal and are situated in semicircular excavations occupying 

 about one-fourth of the entire width of the polypary, though they frequently appear 

 somewhat wider when the polypary is compressed from the side. The character- 

 istic feature of CI. Wilsoni is certainly the presence of the sae or vesicle at the 

 proximal end ; this appears to originate as an expansion of a membrane surround- 

 ing the sicula, which in this species projects from the main body of the polypary 

 to an extraordinary degree. This sac is commonly elliptieal in form, and may 

 measure 10 by 5 mm. (smaller ones are of frequent occurrence), it varies greatly 

 in shape under compression, and is often merely a flattened irregular-shaped body. 



Affinities. — The only other known Climacograptus furnished with a sac at the 

 proximal end is CI. antiquus var. bursifer, but the sac is much closer to the poly- 

 pary in CI. Wilsoni and the characters of the thecas are different. 



Horizon and Local Hies. — Lower Hartfell Shales (zone of CI. Wilsoni). 



S. Scotland : Dobb's Linn ; Hartfell ; Glenkiln Burn ; Black Linn ; Moory 

 Syke, etc., etc. 



Associates, etc. — CI. Wilsoni occurs in great numbers in S. Scotland at the base 

 of the Hartfell Shales, where it marks a well-defined zone. It is commonly asso- 

 ciated with CI. bicomis, CI. Scharenbergi, Dicranog. Nicholsoni. 



Collections. — Geological Survey of Scotland, Lapworth, and the Authors. 



Fig. 129.— Climacograptus Wilsoni, var. There are SOUK) forms of CI. WUsOlli ill wllic 



tubular is, nov. 



Var. tubularis, var. nov. Plate XXVI, tig. 13. 



There are son 

 no sac is developed but in which there occurs a 

 proximal elongated tube similar to that seen in 

 CI. caudatus. These seem to constitute a distinct 

 variety. The characters of the thecse are similar to 

 those of CI. Wilsoni itself. 



Horizon, Localities, etc. — Those of the typical form. 



Proximal end, showing sicula and com- , 



mencement of the tube. On same ( o/lecttOll. Lapworth. 



slab as PI. XXVI, fig. 13. 



GrItOUl' II. 



Clvmacograjpti in which the free vd^c of the theca is slightly inclined, and the 

 apertural margin somewhat introverted. 



Climacograptus antiquus, Lapworth. Plate XXVII, figs. 4 a — e. 



1873. Climacograptus antiquus, Lapworth, Greol. Mag., vol. x, ]>. L34. 



1876. Climacograptus ccelatus, Lapworth, Cat. West Scott. Foss., pi. ii, fig. 56. 



