DIMORPHOGRAPTUS. 351 



The sicula has a length of 2 mm. and the virgella 1 mm.; th. I 1 originates 



slightly above the aperture of the sicula, and grows 



Figs. 228 a and b. — Dimorphoqmntus i n i • i ,n -t i • -i i 



con/ert us, var. Swandoni (Lapw.). upward and very slightly outward, being closely 



adpressed to the sicula for the greater part of its 

 length ; each of the next few thecae develops from 

 the theca immediately below until th. -I 1 or th. 5 1 

 is reached ; from th. -I 1 (or th. 5 1 ) two thecse, th. 5 1 

 (or th. 6 1 ) and th. I 2 are developed, th. I 2 from the 

 back (obverse view) of the initial portion, and th. 5 1 

 (or th. 6 1 ) from the front and apertural region, and 

 a the polypary is thenceforward biserial. 



a. Uniseriai portion, obverse aspect. Each of the biserial theca3 apparently buds from 



Donaghadee, Co. Down ; Birkhill . . . 



shales. Coll. Eiies. the theca next below it belonging to its own series ; 



6. Reverse aspect. Ibid. 



the septum, however, never seems to have been 

 more than partial throughout its entire length. In the earlier part of the biserial 

 portion the thecse are markedly alternate, but they become less so towards the 

 distal end of the polypary. The apertural margins are mainly convex, but tend 

 to become slightly concave towards the exterior, this double curvature being most 

 characteristic. 



Affinities. — When fully developed var. Swanstoni, with its gracefully curved 

 polypary, presents a most characteristic appearance. When young, however, it 

 is liable to be confused with specimens of Monog. gregarius until the biserial 

 portion commences ; the thecse in these two forms are practically identical and 

 both have long sicula?, though that of Monog. gregarius is in reality far the longer. 

 Amongst the Dimorphograpti, var. Swanstoni is certainly most closely allied to 

 Dimorphog. confertus, from which it differs chiefly in its smaller size, more graceful 

 curvature and less closely- set thecse. 



Horizon and Localities. — Lower Birkhill Shales (zones of Cephalog. [?] acuminatus 

 and Orthog. vesiculosus) ; Stockdale Shales (zone of Dimorphog. confertus); Dolgadfan 

 Shales (Dimorjphograptus band). 



S. Scotland : Dobb's Linn, etc. Lake District : Skelgill. Ireland : Coalpit 

 Bay, Donaghadee. G. Wales : Pennant, near Llanbrynmair. 



Associates, etc. — Dimorphog. Swanstoni is a fairly common fossil in the lower 

 beds of the Lower Birkhill Shales of Scotland and their equivalents in other parts 

 of Great Britain and Ireland. It has quite a gregarious habit in the Orthog. 

 vesiculosus zone, and is commonly associated with such forms as 0. vesiculosus, 

 Dimorphog. confertus, Glimacog. normalis, and Monog. tenuis. In the Scottish 

 specimens it is noticeable that the biserial portion commences after th. 4 1 , while in 

 the Irish specimens there are 5 uniseriai thecae. The specimens from Wales are 

 beautifully preserved in full relief, and there is no trace of a septal groove in the 

 reverse aspect. 



