298 BRITISH GRAPT0L1TES. 



aspect the thecae either show as mere crenulations in the ventral margin or seem 

 almost obliterated as the result of compression (PL XXXII, fig. 12 b). It is 

 highly characteristic of this species that when preserved in relief the two ventral 

 sides are never alike in appearance (Figs. 200 d — j). In scalariform views 

 (Figs. 200/ and i) the large size of the apertures compared with that of the free 

 outer edge of the theca is very obvious. These details can only be made out in 

 specimens in low relief, and it is therefore not surprising to find that the 

 different views of this form have been regarded as distinct species by earlier 

 authors ; such forms as Hall's Graptolithus marcidus and Hopkinson's Diplog. 

 Mheridgii are now generally admitted (comp. Ruedemann) to be only differently 

 presented specimens of Gryptog. tricomis. 



Affinities. — Gryptog. tricomis can be readily identified. It is a far more 

 slender and delicate form than any other Cryptograptus yet recognised. 



Horizon and Localities. — Upper Arenig, Llandeilo - Bala, Glenkiln - Hartfell 

 (especially in zones of Dicellog. patidosus and Glimacog. Wilsoni). 



S. Scotland: Hartfell; Dobb's Linn; Tottleham's Burn, Castle Douglas; 

 Berrybush Burn, St. Mary's ; Birnock, Abington ; Laggan Gill, Girvan, etc. 

 Wales : Nant yr Orlof , Penmorfa ; Arenig ; Tiddyndicwm ; Abereiddy Bay ; 

 Porthhayog, Ramsey Island ; Blaen-y-delyn Quarry, near Fishguard. Ireland .- 

 Coalpit Bay, Donaghadee; Ballygrot, Co. Down; Craigavad, Co. Down; Derry- 

 naclough, Connemara. 



Associates, etc. — G. tricomis is an abundant fossil in the Upper Llandeilo and 

 Lower Bala rocks and their equivalents, but it is very rarely well preserved 

 owing to the delicate nature of its test. The best specimens are those from 

 Laggan Gill in the Girvan district, where it occurs in low relief in the 

 transition zone between the Balclatchie and Ardwell Groups ; these forms are 

 invariably small. In the beds of Arenig age it occurs associated with Didymog. 

 bifidus and also with D. extensus. In the Lower Llandeilo it is found at Abereiddy 

 Bay in tolerable abundance associated with Didymog. Murchisoni ; in the higher 

 Llandeilo (Glenkiln) beds it occurs in profusion, associated with Dicranog. ziczac, 

 Dicellog. patulosus, and Glimacog. peltifer ; it is also abundant in the overlying basal 

 zone (Glimacog. Wilsoni) of the Hartfell Shales (Bala), associated Avith Glimacog. 

 Scltarenbergi, and Dicranog. Nicliolsoni, and has also been found in the succeeding 

 zone of Dicranog. Glingani. 



Collections. — Geological Survey of Scotland, Sedgwick Museum, H. B. Muff and 

 R. G. Carruthers, Lapworth, and the Authors. 



