144 



BRITISH GRAPTOLITES. 



Figs. 87 b and c. — Dicellograptus divari- 

 catus (Hall). 



The distal thecse arc T5 mm. in length and are essentially of the same type as 

 those of I). intortus. 



Remarks. — D. divaricatus was originally described by Hall from the Norman's 

 Kill beds of America (Glenkiln) ; and it is a very characteristic species in these beds, 



being readily recognised by its form, the rapid 

 increase in width of the stipes, and the character of 

 the thecse. The typical species is not common 

 in Britain, and such specimens as are referable to 

 it are for the most part poorly preserved. Con- 

 sequently the above description has been wholly 

 drawn up from those from the typical American 

 beds. It is also represented in the Glenkiln beds 

 of Scotland by a well-marked variety to which 

 Lapworth gave the name of var. rigidus. 



Affinities. — ]). divaricatus resembles Dicellog. 

 intortus in the character of its thecse, but differs 

 markedly in the general form of the polypary. 

 From all other species of Dicellograptus it can be distinguished by the shape of 

 the thecse. 



Horizon and Localities. — Glenkiln Shales (Llandeilo). 



S. Scotland: Wanlock Head; Craigmichan Scaurs, etc. Wales: Tiddyndicwm ; 

 Abereiddy Bay. Shropshire: Spy Burn (?). 



Associates, etc. — D. divaricatus is confined to beds of Glenkiln age, and occurs in 

 association with all the other fossils characteristic of those beds. Specimens are in 

 the collections of the Sedgwick Museum and of Lapworth. 



a. Specimen, natural size, from Hall's 



typical district, Normanskill Beds 

 Coll. Lapworth. 



b. Enlargement of distal thecse. 



Var. rigidus, Lapworth. Plate XX, figs. 6 a- 



1876. Dicellograptus divaricatus, Lapworth, Cat. West. Scott. Foss., pi. iv, fig. 86. 



1877. Dicellograptus Mnffatensis, var. divaricatus, Lapworth, G-rapt. Co. Down, pi. vii, fig. 10. 

 1880. Dicellograptus divaricatus, var. rigidus, Lapworth, Ann. Mag. Nat. Hist. [5], vol. v, p. 163, 



pi. v, fig. 20. 



In addition to the typical D. divaricatus, there occurs at the same horizon a 

 much stouter form, to which Lapworth has given the varietal name of var. rigidus. 

 It is readily distinguished by the greater breadth of the stipes at the proximal 

 end, where it has a width of 1 mm. and this is maintained throughout their 

 whole length. The dorsal margins of the stipes are frequently connected at 

 the axil by a chitinous film which extends at least as far as a point opposite the 



