DIPLOGRAPTUS. 255 



extensus), is common in the zone of Didymog. bifidus, and then disappears in the 

 zone of D. Murchisoni. In the zone of D. extensus, it occurs in association with the 

 zone fossil, Tetrag. serra and various Callograpti. In the zone of D. hiruiido, 

 Glyptog. dentatus is very rare, but it is found in abundance in the zone of 

 D. bifidus associated with Didymog. bifidus, D. artas, D. acntidens, Mesog. confertus, 

 and other forms. 



The best preserved British specimens known to us come from South Wales, 

 where the rocks have not been so much cleaved as they are in the Lake District. 

 In some of these Welsh specimens the thecal are not quite so closely set in the 

 proximal region, numbering fifteen rather than eighteen in 10 mm. The examples 

 occurring in the Arenig rocks of Shelve are small and poorly preserved. 



Collections. — Geological Survey of England and Wales, Sedgwick Museum, 

 British Museum (Natural History), Lapworth, and the Authors. 



Var. appendiculatus, Tornquist MS. Plate XXXI, fig. 5. 



1898. Diplograptus appendiculatus, Elles, Quart. Jouru. Geol. Soc, vol. liv, p. 518, fig. 30. 



In addition to the more typical Glyptog. dentatus there occurs in the Skiddaw 

 Slates a form which agrees with it in the essential details of the polypary and 

 thecas, but which is distinguished by the presence of a flattened vesicle proceeding 

 from the distal extremity. This vesicle, which is about 2 cm. in length, resembles 

 that of Orihog. vesiculosa*. 



Horizon and Locality. — Upper Skiddaw Slates. 



Lake District : Outerside. 



Associates, etc. — Var. appendiculatus has up to the present time been recorded 

 only from the Skiddaw Slates. Its associates are unknown. 



Collection. — Sedinvick Museum. 



.-> 



Giioup II. 



Glyptograpti in which the axis of the theca was twisted, the thecse subcircular 

 in section, and the apertural margins undulate ; excavations very distinct. 



Diplograptus (Glyptograptus) sinuatus, Nicholson. Plate XXXI, figs. 6 a — c. 



1869. Diplograpsus sinuatus, Nicholson, Aim. and Mag. Nat. Hist. [4], vol. iv, p. 235, pi. xi, fig. 11. 

 1897. Diplograptus (Glyptograptus) sinuatus, Perner, Grapt. de Boheme, pt. iii, p. 5, pi. ix, figs. 

 9—12, and fig. 3 (text). 



Polypary thick, small, commonly less than 2 cm. in length, narrow for the first 

 few mm. of length, then widening abruptly to a breadth of 1*5 mm., and, 



