328 



BRITISH GRAPTOLITES. 



below. In many cases the outer portions of the processes break up into a minor 

 meshwork which shows here and there traces of a connecting film. These 

 processes are paired, for in certain specimens a second series can be seen, fore- 

 shortened and interior to the first series in position. 



Horizon and Localities. — Llandeilo-Bala, Glenkiln Shales — Lower Hartfell 

 (zone of Climacog. Wilsoni). 



S. Scotland : Hartfell, Dobb's Linn ; Rein Gill, Wandel Water ; Cog Burn, 

 a few yards above junction with Polroisk. Shropshire : Oakwood, Pontesford. 



Associates, etc. — Var. costatus is not a common fossil in beds of Llandeilo agfe, 

 but it is perhaps the commonest of the Thysanograpti present. It is fairly 

 abundant in the Lower Hartfell Shales (zone of 01. Wilsoni). It occurs in 

 Shropshire associated with Climacog. antiquus and Mesog. multidens ; in the Glen- 

 kiln Shales of S. Scotland with Dicranog. rectus, Nemag. pertenuis, and Dicellog. 

 sextans ; and in the Lower Hartfell Shales (zone of Climacog. Wilsoni) with the 

 same fossil. 



Collections. — Lapworth, Geological Survey of Scotland, Mr. P. Benson. 



Figs. 216a — c. — Thysanograptiisretusus(Lajyw.). 





3^ 



Lasiograptus (Thysanograptus) retusus (Lapworth). Plate XXXIV, figs. 3 a— c. 



1880. Lasiograj)tus retusus, Lapworth, Ann. Mag. Nat. Hist. [5], vol. v, p. 175, pi. v, figs. 24 a — d. 



Polypary straight, about 2; 5 cm. in length with average uniform breadth of 

 about 2 mm. Thecas attenuate, sixteen to fourteen in 10 mm.; excavations 

 Avide and deep, with thickened and fibre-like margins ; spines, when pre- 

 served, short and stiff ; marginal lacinia 

 absent. Septal groove zig-zag and well 

 marked. 

 Description. — The polypary quickly attains 

 its maximum width of 2 mm., and afterwards 

 the margins are sub-parallel. The details of 

 the structure of the proximal end are unknown. 

 The thecoe are markedly alternate in their 

 arrangement, and closely resemble those of 

 Amplexog. perexcavatiis, but the apertural and 

 Enlargement of part" of pi. ventral margins are occasionally thickened as 



with an inner wire-like fibre, and also the line 

 of the zig-zag septal groove. In their form 



spicuous thecal spines. Enlargement of . . 



part of specimen on same slab as pi. and mode of thickeninp; the thecas approach 



XXXIV, fig. 3 c. ° rl 



closely to those of Thysanog. Ilorhnessi and its 

 variety costatus, but though ventral spines are present a lacinia is apparently 

 wanting. 



^«^ 



a. Proximal end, showing zig-zag septal groove 



and thickenings of ventral and apertural 



margins. 



XXXIV, fi L 

 6. Distal thecoe, showing virgula. Ibid. 

 c. Distal thecse of wider specimen, probably 



referable to this species, showing con- 



