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BRITISH GRAPTOLITES. 



Fio. 198. — Cephalograptus tubulari- 



formis (Nich.). 



Obverse aspect, showing 1 sicula. En- 

 largement of part of PI. XXXII, 

 fig. 9d. 



The virsrula has a similar course to that described for G. cometa, and the virg-ular 

 tube is often prolonged beyond the distal extremity of the polypary. 



The thecas are alternate in their arrangement, 

 and their length decreases steadily from the proxi- 

 mal to the distal extremity of the polypary; they 

 overlap about two thirds their extent at first, but 

 this amount decreases in a distal direction. They 

 average eight in 10 mm., and are not all collected 

 together near the distal end of the polypary as in 

 Gephalog. cometa, but range down the ventral side 

 of the polypary for fully half its extent. The 

 average inclination of the thecas is about 10° in the 

 middle of the polypary, but the amount of inclina- 

 tion decreases both in the proximal and distal regions. 

 The apertural margins are but slightly everted. 



There are no indications of the presence of a 

 septum. 



Affinities. — This species was one of three included by Nicholson in his original 

 description of Diplograptus tubulariformis. As first pointed out by Elles (' Quart. 

 Journ. Geol. Soc.,' 1897) an examination of his type specimens shows that the 

 specimens from which his figs. 12 and 13 were drawn is the species described by 

 her as Gephalog. petalum, while fig. 14 represents a young Pefalog. folium, and 

 fig. 15 a Gephalog. cometa. As figs. 12 and 13 are the first two figures employed 

 by Nicholson to illustrate his species, his name for it is here revived. 



G. tubulariformis is in all respects an intermediate form between the Gephalo- 

 grapti and the Petalograpti. The mature polypary varies exceedingly in shape, 

 some specimens approaching very nearly to Petalog. folium, others resembling 

 more closely Gephalog. cometa. The characters of the proximal end are always 

 more like those of Gephalog. cometa than those of Petalog. folium, but, on the other 

 hand, the thecoe in the distal portion of the polypary are more like those of the 

 latter than those of the former. It may therefore be readily distinguished from 

 either species. 



Horizon and Localities. — Lowest beds of Upper Birkhill Shales (base of zone of 

 Gephalog. cometa). 



S. Scotland : Duffkinell Burn ; Belcraig Burn ; Dobb's Linn ; Frenchland Burn. 

 Ireland : Coalpit Bay, Donaghadee. 



Associates, etc. — G. tubulariformis is a somewhat rare fossil in the Upper Birk- 

 hill Shales of S. Scotland, where it occurs at the base of the zone of Gephalog. 

 cometa ; it is rather more abundant at the same horizon in the shales of Coalpit Bay. 

 Collections. — British Museum (Natural History) (Nicholson's original specimens), 

 Sedgwick Museum, and the Authors. 



