128 BRITISH GRAPTOLITES. 



appearance of the proximal end is very symmetrical. The apertures of th. I 1 and 

 th.Pare distinctly mncronate, a feature which is not observable in any of the 

 later developed thecae. These appear to develop in the normal manner for the 

 most part, but the interthecal wall between the two first thecae developed on 

 either side of the sicula seems, in some of the adult forms, to have grown 

 completely back to the sicular wall. 



The thecae on the main stipes are hard to distinguish, as they are frequently 

 concealed except at the distal extremities of the stipes. To this fact is probably 

 due the view of the earlier palaeontologists that the main stipes were non- 

 polypiferous. The general tendency seems to be for them to appear in preservation 

 on the outer side of the curve of each stipe which is turned in the upward direction, 

 but in the case of the stipe turned in the downward direction they show sometimes 

 on the inner side of the curve and sometimes on the outer side. Their position on 

 the inner side of the curve is probably the normal one, but on the polypary coming 

 to rest the presence of the secondary branches seems to have produced slight torsion 

 of this downward-growing stipe and tended to drag round the thecal-bearing 

 margin to the outer side of the curve, hence in preservation the thecae are frequently 

 seen in such a position. Additional evidence of such torsion can sometimes be 

 detected near the proximal end where the first theca of the downward-growing 

 stipe is seen on the under side in its normal position ; but where the first secondary 

 branch is given off there is an obvious twist of the main stipe, and the second 

 theca appears on the opposite side of the stipe to the first. 



The thecae are of the normal Leptograptid type 



Figs. 76 6 and c— Nemagraptus gracilis . 1 ^ l 



(Hall). on both the main stipe and the secondary branches, 



though the ventral (marginal) excavation is some- 



j-'~™r^ m —rfj^' what small. The theca? on the main stipes are 



slightly more remote than those on the secondary 



branches — seven as compared with eight in 10 mm. ; 



c the amount of overlap is also less, the thecae on the 



i, Thecse of one of the branches in low mam s tipe being only iust in contact, while on the 



relief. Enlargement of part of PI. x o j j 



xix 3 fig. i a. secondary branches the overlap may be as much as 



c. Ditto. ... 



one third their entire length. 

 Affinities. — The typical form of N. gracilis is allied to var. remotus : it may, 

 however, be readily distinguished by the number and position of its secondary 

 branches. From all other Nemagrapti it differs in the direction of growth taken 

 by the two main stipes, i. e. in the presence of the sigmoid bend. 

 Horizon and Localities. — Upper Llandeilo (Glenkiln). 



Wales : Tiddyndicwm ; Builth Road; Porth Badrig, Anglesea. Shropshire: 

 Spy l)iini ; Holywell Burn, etc. 8. Scotland: Belcraig Burn; Glenkiln Burn; 

 Rein Gill ; Berrybush Burn ; Tributary of Crawick Water, Sanquhar ; Bail Hill ; 

 Rough Gill; Eawkwood Burn; Gair Gill; Riddenlees; Fingland Burn; Cowie's 



