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178 BRITISH GRAPTOLITES. 



curvature. Thecse short, fourteen to ten in 10 mm., overlapping one third to 

 one half their length, of the general type of those of Dicellog. elegans. 

 Description. — The biserial portion of the polypary is approximately constant in 

 size and is usually made up of 4 or 5 thecse on each side. There is, however, 

 great variability in the amount and the direction of curvature of the uniserial 

 stipes, and this curvature is always accompanied by a certain amount of torsion. 

 Diverging at first to include between them an axillary angle of about 90°, they 

 commonly grow outward for a length of 7 — 10 mm. and then bend sharply round 

 towards each other in a direction nearly perpendicular to that of their original 

 „,,„«. ± growth; they next grow towards each other until they 



JblU. 113. — Dicranuij nipt us ziczac, ° d ° 



Lapw. ar e almost in contact, when they usually bend away 



/ ^^V' again at 90° without crossing (figs, o a, b). Sometimes, 



'^"^ Jjtef however, they cross each other (fig. 3 c), and occa- 

 sionally they even interlock before growing apart again 



"$|P (fig. o d). In the most perfect specimens known to us 



~" r~ they are seen to grow towards each other for a second 



Complete specimen in full relief, time and eventually to meet (fig. 3 d). Specimens with 



with unusually short biserial u ° ' _ L 



portion. Dob'b's Linn, Glen- three bends are very rare: those showing two bends (or 



kiln Shales. Coll. Lapworth. J b V 



a Z form) are of common occurrence. 



The sicula has not been observed by us, though its strong apertural spine can 

 frequently be detected at the extremity of the proximal portion of the polypary. 

 The development of the thecse is essentially similar to that in all other Dicrano- 

 grapti. It is doubtful, however, if there is a septum in the biserial portion. 



Mesial spines are given off from the outer wall of all the thecse of the biserial 

 portion. The proximal thecse have a length of about 1*2 mm., and it is possible 

 that they are longer in the more distal portions of the polypary ; but this cannot be 

 determined with accuracy, since the uniserial stipes appear to be always twisted, 

 and their thecse are therefore seen in profile only for very short distances. 



Affinities. — I), ziczac is easily distinguished from all other British Dicrano- 

 grapti by the peculiar curvature of the uniserial stipes. It is somewhat similar 

 in form to Dicranog. furcatus of Hall, but differs from it in the more marked 

 " ziczac" curvature of the stipes and the greater length of the biserial portion. 



Horizon and Localities. — Glenkiln Shales. 



8. Scotland .- Dobb's Linn ; Belcraig Burn ; Glenkiln Burn ; Morroch Bay ; 

 Berrybush Burn ; Craigmichan Scaurs, etc. Ireland : Ballygrot. N. Wales : 

 Tiddyudicwm. 



Associates, etc. — D. ziczac occurs in some abundance in the Glenkiln Shales of 

 S. Scotland. It has also been recognised in Ireland and N. Wales. In all these 

 localities it occurs associated with Nema.g. gracilis, Didymog. saperstes, Dicellog. 

 sextans, Dicellog. patulosus, Dicellog. intortus, Dvplog. Wlbitfieldi, Hallog. bimucronatus, 

 Thamnograptus, etc. The most complete specimens known to us were collected by 



