RETIOGRAPTUS. 317 



in shape. In examples preserved in partial relief (Fig. 209 a) the nearer composite 

 ventral strands form the outer visible edges of the fossil, and appear to be formed 

 in part of the thickened ventral edges of the successive thecas and in part of their 

 apertural lists ; these last when compressed give the profile a spinose appearance. 

 Sometimes a part or the whole of one of the ventral lattices is shown in this view 

 (Fig. 209 a). The ventral lattice itself sometimes becomes wholly detached (Fig. 

 209 c) and its undulating limiting strands and the cross-bars are well exhibited. 



When the fossil is in low relief (Fig. 209 a) the continuous attenuated test is 

 usually found preserved in the earlier parts of the polypary : when greatly 

 compressed little or nothing remains except the cage-like clathria (Fig. 209 b). 



The transverse section of the polypary appears to have been subquadrangular 

 or truncato-elliptical. The thecae were somewhat of the form of those of Ortlio- 

 graptus, but broad towards their bases and narrowing in the direction of the 

 aperture, which is strongly listed. The thecas were distinctly inclined; the 

 proximal or ventral wall of each was somewhat sigmoid, swelling out below and 

 bent inwards above. There are occasional traces of a short apertural mucro, but 

 in this species no evidences of the presence of septal spurs or processes have been 

 detected. 



The sicula is only visible in its apertural portion, Avhich extends outside the 

 polypary, and is covered by a continuous membranous test. 



Affinities. — lletiog. Geinitzianus is very distinct in its outward features from 

 any of the other British species of the Glossograptidse. It most nearly resembles 

 R. aculeatus, which is, however, a less compact and more straggling form. 



Horizon and Localities. — Llandeilo, Glenkiln Shales. 



8. Scotland .- Birnock Water ; Head of Wandel Burn, Birnock Water ; Berry- 

 bush Barn; Benan Burn, R. Stinchar. G. Wales : Gwern-y-fed-fach, near Builth. 



Associates, etc. — Betiog. Geinitzianus is a rare fossil in the Glenkiln Shales and 

 other beds on the same horizon, but when found is commonly associated with 

 Dicellog. sextans, Didymog. superstes, Nemag. pertenuis, N. gracilis, Climacog. 

 antiquus, etc. 



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



Retiograptus aculeatus (Lapworth). 



1880. Idiograptus aculeatus, Lapworth, Ann. Mag. Nat. Hist. [5], vol. v, p. 168, pi. v, figs. 23 a — f. 



Such specimens of this form as we have in our possession are in their present 

 state too poorly preserved to enable us to make out the necessary details for 

 figuring and description, and reference may be made to Lapworth's original 

 diagnosis and illustrations (loc. cit. supra). 



