27G 



BRITISH GRAPTOLITES. 



^IGE 



!. 189 



a — c- 



-Petoi 



logr 



apt us 



palmeus, 







var. 



latus. 



Barr. 





/, 





/ / A 



/ i ^ 















■ 



/] 



1 



/A 







■ 



i 



In addition to the typical form, V. palmeus, there occurs in the Middle 

 Birkhill Shales a variety which differs in (1) its greater width; (2) the greater 

 number of thecse in the same unit of length ; (3) the steeper inclination of the 

 thecse and their greater curvature. 



The polypary is commonly shorter than in Petalog. palmeus s.s. ; it is usually 



about 1"3 cm. in length. The maximum width of 

 4 — 5 mm. is attained at once and maintained to the 

 distal extremity, which is broadly truncate, but 

 occasionally rounded. 



The thecre earliest developed are more curved 

 than those of the typical form and are rather 

 longer; the other thecae have an average length of 

 fully 2 mm., and are three times as long as wide. 

 The thecse in general average fourteen to sixteen 

 in 10 mm. At the proximal end they are inclined 

 at 45°, but in the more mature parts of the poly- 

 pary curve very distinctly in an outward direction, 

 and the angle of inclination decreases uniformly 

 from the proximal up to the distal end, where it is 

 about 20°. 



Horizon and Localities. — Birkhill Shales (Middle). 

 S. Scotland: Dobb's Linn; Garple Linn; and 

 wherever the zone of Monog. gregarius is typically 

 developed. Lake District .- Skelgill. Ireland : Coalpit Bay, Donaghadee ; Mullagh- 

 nabuoyah, Pomeroy. Wales .- Pary's Mountain, Anglesea. 



Associates, etc. — Var. latus occurs in abundance in certain beds of the Birkhill 

 Shales ; it makes its first appearance about the middle of the zone of Monog. 

 gregarius and is most abundant throughout the middle and upper parts of that 

 zone. It never ranges up into the Gala Series, so far as we are aware. It is 

 commonly associated with Monog. gregarius, M. convolntus, M. fimbrlatus, Glyptog. 

 tamariscus, etc. 



Collections. — British Museum (Natural History), Sedgwick Museum, Lapworth, 

 and the Authors. 



a. Obverse aspect, showing sicula. Long 



Linn, Dobb's Linn ; Birkhill Shales 

 (zone of M. gregarius). Coll. Elles. 



b. Reverse aspect, showing virgella. 



Ibid. 



c. Transverse section, showing septum, 



etc. Skelgill, Skelgill Shales. 

 Figured, Elles, Quart. Journ. Geol. 

 Soc.,liii,pl. xiv, fig. 14 (as var. ovato- 

 elongatus). 



Var. tenuis, Barrande. Plate XXXII, figs. 3 a — d. 



1850. Graptolitlius palmeus var. tenuis, Barrande, Grrapt. de Bolieme, p. 61, pi. iii, figs. 1 and 2. 

 1897. Petalograptus palmeus var. tenuis, Elles, Quart. Journ. Geol. Soc, vol. liii, p. 196, pi. xiv, 

 figs. 9, 10. 



In addition to the wide variety of Petalog. palmeus last mentioned, there is a 

 second variety which differs from the typical form — (1) in its extreme narrowness; 



