140 



BRITISH GRAPTOLITES. 



Figs. 84 d and e. — Dicellograptus 

 planatus, Lapw. 



Th. 2 3 is apparently developed from the under side of th. 2 1 . Each of the 

 remaining thecae is developed from the one immediately below it. 



The thecae throughout the whole length of the stipes approximate in form 

 to those of a typical Glimacograptus, their free ventral walls being parallel to 



the dorsal margin of the polypary and their aper- 

 tures perpendicular to it. The mature thecae have 

 an average length of 2 mm. The variable appear- 

 ance of the apertures when compressed suggests 

 introtorsion of the apertural region of the thecae. 



Affinities. — D. comjplanatus resembles D. For- 

 chammeri most nearly in general form, but it differs 

 from it markedly in the characters of the proximal 

 extremity and also in those of the thecae. 



Horizon and Localities. — Upper Hartfell Shales. 

 In the Moffat area D. complanatus occurs at a 

 single horizon in the Upper Hartfell Shales, being 

 restricted, so far as known, to a thin band of black 

 shale near the base of the so-called " Barren Mud- 

 stones." It occurs also in a corresponding zone in 

 the Whitehouse Beds (Bala) of Girvan. 

 Dobb's Linn; Moory Sike. Portpatrick. Girvan 

 inland: Co. Down, Coalpit Bay. 



D. comjplanatus occurs in S. Scotland associated with some 

 small Diplograpti — JDijplog. socialis, etc. The best specimens known are in the 

 collections of Lapworth and the Authors. 



d. Proximal thecae, profile view. Dobb's 



Linn, Hartfell Shales. Coll. Lap- 

 worth. 



e. Distal thecae, scalariform view. Same 



stab as Fig. 84 d. 



8. Scotland, Moffat area : 

 area : Myoch Bay 

 Associates, etc. 



Var. ornatus, var. nov. Plate XX, figs. 2 a- 



In addition to the typical form of Dicellog. convplanatus, there occurs in S. 

 Scotland a slender variety which is characterised by an extraordinary development 

 of the lateral spines. Its close alliance to Dicellog. comjplanatus is, however, 

 evident in the general shape of the polypary and the characters of the 

 thecae. 



Descrijrtion. — The stipes widen from about '3— *4 mm. at their origin to about 

 "7 mm. distally ; they are usually rigid, but occasionally show slight curvature. 

 The axil is characteristically wide open and square at the base. 



The sicula probably exceeds 1 mm. in length, but complete specimens have 

 not been obtained. Th. I 1 and th. I 2 appear to be always somewhat abnormal; 

 they show no curvature of their ventral walls, and grow straight out in a 



