CEPHALOGRAPTUS. 287 



Affinities. — The distinctive characters of the external form of this species led 

 Hopkinson to found for its inclusion the sub-genus Gephalograptus, in order to 

 separate it from the ordinary types of Diplograpti, though he did not recognise all 

 its structural characteristics, which were fully worked out later by Tornquist. 



G. cometa may be readily distinguished from all other Petal ograpti except 

 Gephalog. tubular if ormis by the greatly protracted character of its proximal portion. 

 From G. tubulariformis, which is also proximally prolonged, it may be distinguished 

 by its fewer thecas, the apertures of which are arranged so as to form a kind of 

 crown at the distal extremity. 



Horizon and Localities. — Base of the Upper Birkhill Shales, zone of Gephalog. 

 cometa, immediately below that of Monog. spinigerus. 



N. Wales : Pary's Mountain, Angiesea. Lake District .- Browgill. S. Scotland : 

 Dobb's Linn; Belcraig Burn; Hartf ell ; Duffkinell Burn; etc. Ireland: Coal- 

 pit Bay, Donaghadee. 



Associates, etc. — C. cometa occurs abundantly in a well-defined though narrow 

 band immediately below the zone of Monog. spinigerus in S. Scotland, the Lake 

 District, and Ireland, and is commonly associated with Glyptog. tamariscus, 

 G. sinuatus, Monog. convolutus, M. lohiferus, M. gregarius, and M. jaculum. 



Collections.— -British Museum (Natural History), Sedgwick Museum, Mr. G. J. 

 Williams of Bangor, Lapworth, and the Authors. 



Cephalograptus tubulariformis (Nicholson). Plate XXXII, figs. 9 a—d. 



1867. Diplograpsns tubulariformis, Nicholson, Geol. Mag., vol. iv, pi. vii, figs. 12, 13. 



1897. Cephalograptus petalum, Elles, Quart. Journ. Geol. Soc, vol. liii, p. 206, pi. xiii, figs. 6—9. 



Polypary wedge-shaped, from 1 — 2*5 cm. in length, very narrow proximally 



but widening quickly to a maximum breadth of 4 mm., which is attained 



opposite the apertures of the third pair of thecae and thence maintained up 



to the distal extremity. Sicula small. Thecse about eight in 10 mm., 



alternate, long straight tubes from 9 — 6 mm. in length, and about eight 



times as long as wide, inclined at 10° ; apertural margins normal. 



Description. — The whole proximal end is slender and protracted, but to a much 



lesser degree than in Gephalog. cometa, and it is usually quite straight. The position 



of the sicula strongly recalls that in the Monograpti, being entirely free on one 



side in both aspects of the polypary. It has a length of about 1*5 mm., and 



the virgella, which is sometimes visible, may measure at least 3 mm. in length. 



Th. l l arises slightly above the aperture of the sicula, and describing an outward 



curve attains a length of about 9 mm.; th. 1~ develops from th. I 1 immediately 



after it has gone beyond the level of the apex of the sicula. 



