170 



BRITISH GRAPTOLITES. 



Figs. 106 a and b. — Dicranograptus 

 rectus, Hoi>k. 





'-' ) 



f 



proximal parts of the uniserial stipes, but not on the distal portions so far as we 



have been able to observe. As a general rule, the free outer walls of the thecse in 



the biserial portion of the polypary appear to be 

 more curved than those on the uniserial portions, but 

 this is probably due to their mesial spines. 



Affinities. — In general form, with its short biserial 

 portion and long uniserial stipes, D. rectus perhaps 

 most closely resembles D. Nicholsoni, with which it 

 also agrees in showing marked increase in width 

 throughout the biserial portion. It differs from 

 this species, however, in the characteristic straight 

 line of the ventral margin throughout the whole 

 polypary, there being no break like that in D. 

 Nicholsoni where the uniserial stipes diverge. Small 

 forms of JK rectus bear a certain resemblance to B. 

 tardiusculus and J). Glingani, but in D. tardiusculus 

 the biserial portion is longer. The thecas of D. 



Glingani sire of a very different type. 



Horizon a ml Lucidities. — Llandeilo (Leptograjptus Beds), Upper Glenkiln Shales, 



Lower Hartfell Shales. 



S. Scotland: Laggan Gill, Glengonnar Water ; Glenkiln Burn; Dobb's Linn; 



Rein Gill; Wandel Water; Loch Ryan. Shropshire: Spy Burn. A. Wales : 



Tiddyndicwm. 



Associates, etc. — D. rectus is a fairly abundant fossil at someS. Scottish localities 



in the higher beds of the Glenkiln Shales, where it has been found in association 



with Dicellog. sextans, Dicranog. brevicaulis, Diplog. Whitfieldi, Glossog. Einchsii, and 



Viplograjpti of the "foliaceus" type. It is a rare fossil in the zone of Glimacog. 



Wilsoni, at the base of the Hartfell Shales, but it has been found there in association 



with the zone fossil. 



The best specimens are in the collection of the Geological Survey of Scotland 



and those of Lapworth and G. L. Elles. 



Biserial portion showing mesial 

 spines. Enlargement of part of 

 PI. XXIV, fig-. 4 a. 



Uniserial stipe. Ibid. 



Dicranograptus celticus, sp. 



Plate XXIV, figs. 5 a, b, 



Polypary consisting of a short biserial portion about 4 mm. in extent, and two 

 straight uniserial stipes of a length of 6 — 7 cm., which widen throughout ; 

 axillary angle 30° — 40°. Thecas eighteen to twelve in 10 mm., with free 

 outer wall slightly curved, and introverted apertures opening within a 

 narrow excavation which occupies less than one third the width of the stipe. 



Description, — The biserial portion of the polypary is usually composed of G to 8 



