350 



BRITISH GRAPTOLITES. 



Figs. 227 a and b. — Dim6rphograptv.s 

 confertus (Nieh.). 



•v-v 

 jC *, X 



gradually outward; from th. 3 1 two thecae are developed, th. I 2 from the initial 

 pari of th. 3 1 and th. 4 1 from its apertural region. 



The thecae of the biserial portion are of the 

 general Orthograptus type, and the margins are 

 typically undulate. The uniserial thecae are more 

 distant than those of the biserial portion, approxi- 

 mating in form to those of Monograptus gregarius. 

 Their apertural margins are rounded and the 

 denticles are somewhat rostrate. 



Affinities. — I), confertus resembles closely its var. 

 Siva ii sto ui, but may be distinguished by its shorter 

 and stiffer uniserial portion, and longer and 

 straighter biserial portion ; the thecae also are more 

 closely set. 



Horizon and Localities. — Stockdale Shales (zone 

 of Dimorphog. confertus), Lower Birkhill Shales 

 (zone of Orihog. vesiculosus). 



Lake District: Skelgill. 8. Scotland: Dobb's 

 Linn. N.E. Ireland : Mill Sluice below Slate Quarry, Tirnaskea, Co. Tyrone. 



Associates, etc. — D. confertus is an abundant fossil in the beds near the base of 

 the Stockdale Shales at Skelgill, where it forms a well-recognised zone. The fossils 

 from this locality are, however, very much distorted by cleavage and the characters 

 of the thecae are hard to determine. Much better specimens have been obtained 

 from Scotland and Ireland, and from these our description has been drawn up. 

 It will be noticed that some of the characteristics of the species as given above 

 differ from those given by Nicholson in his original description, but this is 

 accounted for by the better preserved material subsequently collected. 



Collections. — Sedgwick Museum, Marr, Geological Survey of Scotland, 

 Lapworth, and the Authors. 



a. Obverse aspect, showing sicnla and 



form of thecae. Enlargement of 

 part of PI. XXXV, fig. 3 d. 



b. Obverse aspect, showing sicula and 



virgella. Enlargement of part of 

 PI. XXXV, fig. 3 c. 



Var. Swanstoni (Lapworth). Plate XXXV, figs. 4a — /. 



1852. Diplograptus dentatvs, Geiuitz, Die Graptolithen, p. 23, pi. i, figs. 25 a and b. 



1876. Dimorphograptus Swanstoni, Lapworth, Geol. Mag., p. 548, pi. xx, figs. 13 a—c. 



1877. Dimorphograptus Swanstoni, Lapworth, Proc. Belfast Nat. Field Club, p. 131, pi. vi, fig. 5. 



Var. Swanstoni differs from the typical species (1) in its smaller size, for it 

 rarely exceeds 1 cm. in length ; (2) the graceful curvature of the polypary ; (3) the 

 longer uniserial portion, which is composed of 5 thecae; and (1) the greater 

 distance of the thecae from each other even in the biserial portion. 



