RETIOLITBS. 



341 



Figs. 222 a, b, and c — Plegmatograptus 

 nebula, sp. nov. 











appearances according to the direction of compression. In the bi-profile view, 

 the test of the polypary seems at first sight to be continuous and to have a 



nebulous and delicately membranous appearance, 

 but when magnified it is seen to be very minutely 

 reticulate ; the component fibres are hardly dis- 

 tinguishable even under the lens in some examples, 

 though in others the reticulation is distinctly shown. 

 The ventral margins of the polypary are bordered 

 by a continuous lacinia very similar to that of 

 ThysdTiograptus Harhnessi, var. costatus ; but the 

 outer threads anastomose more frequently, and 

 occasionally appear as if covered by a faint mem- 

 branous film. 



In what appears to be the scalariforin view, the 

 whole surface occupied by the fossil is covered by a 

 network of threads much coarser and more definite 

 and with meshes larger than those of the fine 

 reticula above mentioned. This network may be 

 interpreted as representing both ventral lacinia? as 

 shown superposed when the polypary has been com- 

 pressed from the ventral aspect. 



A virgella is occasionally visible, and in one 

 specimen has a length of o mm. What appears to 

 be the virgula is occasionally seen distally prolonged 

 beyond the polypary. 

 The thecae number twelve in 10 mm., but their shapes are very obscure. 

 We figure (PI. XXXTV, fig. 14 d) a larger and wider form which we pro- 

 visionally refer to this species. If more specimens are found this may eventually 

 be separated off as a distinct variety. 



Affinities. — The affinities of P. nebula appear to be on the one hand with 

 Thysanograptus Harhnessi, var. costatus, and on the other with Plegmatog. obesus, 

 but its peculiar characters distinguish it readily from both. 



Horizon and Localities. — Hartfell Shales (zone of Dicranog. Glingani). 

 8. Scotland : Dobb's Linn ; Hartfell ; N. end Morrach Bay, Portpatrick ; 

 Auchenvey Burn, near Ford Corsock ; Burn at Gordonstown, half mile E.N.E. of 

 Dairy; Wood of Cree, Newton Stewart; Clodderoch Burn, above footpath. 



Associates, etc. — Plegmatog. nebula is a very common Ttetiolite in the Lower 

 Hartfell Shales of some localities in S. Scotland, especially in the zone of Dicranog. 

 Glingani, where it occurs associated with Dicellog. Morrisi, D. Forchammeri, Oiihog. 

 calcaratus, var. vulgatus, and Lasiog. margaritatus. 



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



a. Small specimen, as seen in bi-profile 

 view, showing complete lacinia. 

 N. end of Morrach Bay, Portpatrick ; 

 Hartfell Shales. Geol. Survey of 

 Scotland. 



6. Complete specimen as seen in scalari- 

 forin view. Ibid. 



e. Specimen in bi-profile view. Hart- 

 fell (?), Hartfell Shales. Coll. Lap- 

 worth. 



