DIPLOCIRAPTUS. 229 



might be confused is CI. innotatus ; in this species, however, the characters of the 

 thecae are quite different, while the spine is not apertural, but mesial in position. 



Horizon and Localities. — Llandovery (Birkhill Shales), zone of Monog. gregarius. 



S. Scotland : Dobb's Linn. Ireland: Ballygrot (?). 



Associates, etc. — Orthog. insectiformis is a somewhat rare fossil in the Birkhill 

 Shales of S. Scotland, where it occurs in the zone of Monog. gregarius with CI. 

 Tomquisii and ill", gregarius. 



Collections. — The Geological Survey of Scotland, British Museum (Natural 

 History), Lapworth, and the Authors. 



Diplograptus (Orthograptus) vesiculosus, Xich. Plate XXVIII, figs. 8 a — d. 



1868. Diplotjrcqisus vesiculosus, Nicholson, Ami. Mag. Nat. Hist. [4], vol. i, pi. iii, fig. 11. 



1869. Diplograpsus vesiculosus, Nicholson, Ami. Mag. Nat. Hist. [4], vol. iv, p. 237, pi. xi, figs. 14, 15. 

 1874. Diplograpsus vesiculosus, Dairon, Trans. Geol. Soc. Glasgow, p. 183, pi. i, fig. 20. 



1876. Diplograjrtus vesiculosus, Lapworth, Cat. West. Scott. Foss., pi. ii, fig. 41. 



1877. Diplograptus vesiculosus, Lapworth, Grapt. Co. Down, pi. vi, fig. 19. 



Polypary robust, from 3 — 6 cm. in length with a uniform breadth of 3 — 4 mm., 

 and having a characteristic vesicle prolonged beyond the distal extremity. 

 Sicula very large, with long and conspicuous virgella. Thecas broad, ten 

 to eight in 10 mm., of the general Orthograptus type, expanding towards 

 aperture, with a length of 2 mm., and overlapping for one half their extent. 

 Apertural margins but slightly lobate, wide. 

 Description. — 0. vesiculosus is broad even in its initial stages. Its most con- 

 spicuous feature is the presence of a tubular membranous body within the polypary ; 

 Figs. 151 a— c— Orthograptus vesicu- which expands, on leaving the distal extremity, 



losus, Nicholson. . , „ . „ , . , . , . , 



into a large tusitorm, ovate or cylindrical vesicle, 

 with an acuminate apex, which is bounded by strong 

 filiform margins; sometimes it emerges from the 

 polypary quite small, and gradually swells out and 

 then again diminishes in size ; at other times it 

 begins to distend before leaving the polypary, and 

 is clearly visible inside it, but it always attains its 

 maximum size outside, and may be as much as 

 5 cm. in length, with a breadth of 1 cm. What 

 a b c appears to be the virgula may frequently be 



various stages m the development of detected running through it. This membranous 



the yoiing polypary, showing great o o 



length of sicuia and origin of th i'. tu jj e appears to originate as a film surrounding the 



o. Dobb s Linn, Lower Bn-khill Shales. 





Coll. Lapworth. sicula and is present from the earliest stages of 



b. Ibid. i _ ° 



c. On same slab as 150 a. growth of the polypary (see Figs. 151 a — -/). 



