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DICRANOGRAPTUS. 171 



thecas on eacli side, and widens from 1 mm. at its origin to about 2 mm. at the 

 axil. The uniserial stipes arc of great length and are typically straight ; this 

 width increases (exclusive of spines) from 1 nun. at their origin to 1*4 mm. near 

 their distal cm a Is. 



The sicula is unknown, but its apertural spine is 



Figs. 107 a and 6. — Dicranograptus celti- , , , -, r , ,, .-, „ ., 



cus, sp. iiov. stout; the earliest thecae are rather smaller than 



those subsequently developed. 



All those in the biserial portion appear to be 

 closely set (this may lie partly due to compression) 

 and are distinctly alternate in their arrangement; 

 they are provided with spines, as are also the 

 proximal thecse of the uniserial stipes. The mature 

 thecae have an average length of 2 mm. and overlap 

 for about half their extent. 



Affinities. — IK celticus bears some resemblance 

 to D. Nicholsoni and D. rectus in general form. It 



a. Biserial portion showing spines. -..^ £ 



Enlargement of part of PL xxiv, cutters from V. JSichotsom, however, in the closely 



b. Uniserial stipe, ibid. set thecae of the stem, in the widening f the uni- 



serial stipes, and in the simpler character of the 

 theca? ; and from D. rectus in the first two of these characters, and also in the 

 absence of the unbroken line of the ventral margin of the polypary. 



Horizon and Localities. — Glenkiln Shales. 



8. Scotland: Rein Gill, Wandel Water; Dobb's Linn ; Glentewing Burn, etc. 



Associates, etc. — D. celticus is a fairly abundant form at certain localities in 

 the higher beds of the Glenkiln Shales, where it occurs associated with Vicellog. 

 sextans, Leptog. flaccidus, Vicranog. for-mosus, Vicranog. rectus, and certain 

 Vvplograpti. 



The best specimens are in the collections of the British Museum of Natural 

 History, Geological Survey of Scotland, and that of G. L. Elles. 



Group IV. — Type Dicranog. Nicholsoni. 



Dicranograpti in which the thecaa have strongly curved ventral walls and 

 markedly introverted and introtorted apertures. 



Dicranograptus Nicholsoni, Hopkinson. Plate XXV, figs. 1 a — //. 



1870. Dicranograptus Nicholsoni, Hopkinson, Geol. Mag., vol. vii, p. 357, pi. xvi, fig. 3. 



1876. Dicranograptus Nicholsoni, Lapworth, Cat. West. Scott. Poss., pi. iii, fig. 79. 



1877. Dicranograptus Nicholsoni , Lapworth, Q-rapt. co. Down, Proc. Belfast Nat. Field Club. p. 141, 

 pi. vii, fig. 2. 



