310 BRITISH GRAPTOLITES. 



vertically downward. Virgular tube long and conspicuous. Thecae sixteen 



to ten in 10 mm., overlapping about one half their length; apertural 



margins straight, or but slightly everted; apertural spines strong, arcuate, 



nearly as long as width of polypary ; septal spines straight, spur-like, 



ascending. 



Description. — The polypary of Glossog. HincJcsii varies very much in size ; some 



specimens may be as much as 4 cm. in length, but the majority are smaller and do 



not exceed 2*5 cm. These smaller forms rarely reach the maximum breadth, but 



all show the subparallel character of the ventral margins. 



The characters of the proximal end are obscure ; the sicula seems to be at least 

 1 mm. in length and to have a stout virgella and also an additional apertural spine on 



the other side of its aperture, while 



Figs. 205 a — c. — Glossograptus Hincksii (Hopk.). 



the apertural portion lies wholly 

 outside the polypary. The position 

 on the sicula where th. I 1 originates 

 has not been determined, but the 

 growth of th. I 1 is mainly in a hori- 

 zontal direction, the thecae bending 

 upward eventually close to the 

 aperture ; the same is also the case 

 with th. I 3 . Spines are given off 



a. Proximal end, showing both thecal and septal spines. En- 



largement of part of pi. xxxin, fig-. 2/. from these thecas both from the 



6. Proximal end, showing long curved apertural spines. En- 

 largement of part of pi. xxxin, fig. 2 a. aperture and also from the point 



c. Young specimen, probably referable to this form, with . . . i • i i 



conspicuous septal spine. Dobb's Linn, Glenkiln Shales. (mesial) at AvhlCll change ot growth 



apw01 takes place, and they are directed 



mainly downward. The second pair of thecae, th. 2 1 and th. 2~, and all succeeding 

 thecae grow upward and outward in the usual Diplograptid manner. The apertural 

 spines become gradually more and more ascending in position, until in the 

 distal part of the polypary they are directed upward. These spines are 

 always strong, and are arcuate in form ; in the proximal region they do not 

 exceed 1 mm. in length, but in the median region of the polypary, where they 

 are practically horizontal, they may be nearly 3 mm. long. The septal spines are 

 straighter and spur-like, and appear to have an ascending direction. 



The thickenings of the apertural margins (apertural lists) and of the ventral 

 margins are occasionally well shown, and certain specimens show the thickening 

 of the outer edge of the septum almost individualised as a continuous band or 

 thread (Fig. 205 e). 



The virgula is very conspicuous and is generally prolonged for a distance 

 greater than that of the length of the polypary. In the Glenkiln Shales of 

 Wanlock Water the virgula appears as a definite wire-like thread on the 

 surface of the rock, but at Birnock Water and Hartfell, etc., it seems to be 



