32G BRITISH GEAPTOLITES. 



width of the central portion ; apparently continuous and complete, but 

 outer portions rarely preserved. 

 Description. — The polypary is very small, but relatively broad ; it attains its 

 maximum width of 2 mm. rapidly, and thereafter the margins are parallel. 



Specimens preserved in the scalariform view are 



Figs. 214 a and b. — Thysanograptus n i in -i 



Harknessi (Nieh.). ' generally much narrower than those presented m 



bi-profile view. 



The sicula has a length of 1"5 mm., and in well 

 preserved specimens three pairs of spines are 

 commonly visible at the proximal end ; one pair 

 arising, as in Glossograptus, from each side of 

 the aperture of the sicula, and one pair from the 

 mesial angles of each of the thecas — th. I 1 and th. I 2 . 

 Th. I 1 grows at first horizontally, but soon bends 

 ° Y 3* and three Sec? S C ° mplete upward in the direction of the aperture, while in 



6. Complete specimen showing clathria ^ p t | ie „ r0W tll is Upward and outward throughout, 

 partly developed. .Reverse side ol o i o 



specimen figured pi. xxxiv, fig. ^he external processes are delicate and thin. 



la. l 



They are all ventral, and are apparently mesial in 

 origin, arising from the thecal walls at the outer angle of the sigmoid bend. 

 There are two belonging to each theca, and by the fusion of their anastomosing 

 terminations form the marginal meshwork surrounding the ventral edges of 

 the polypary. This lacinia is now usually very fragmentary, but its completeness 

 in occasional examples shows that it must have been originally continuous. 



Affinities. — Thysanog. HarJcnessi agrees very closely with its variety costatus 

 in the general type of its thecae, and Lapworth at one time held that the two 

 forms were identical. But in T. Harknessi the polypary is always smaller than in 

 var. costatus, the breadth is more uniform, and the thecge are more remote. It is 

 desirable, therefore, that the two forms be kept apart, and we retain Lasicg. costatus 

 as a variety. 



Horizon caul Localities. — Bala, Hartfell Shales, Upper Dicranogra/ptus Shales. 



S.Scotland: Hartfell; above footpath Clodderoch Burn. S.Wales: Between 

 Pemblewin and Stoneyford, Pembrokeshire. 



Associates, etc. — Thysanog. Harlniessi occurs in the Hartfell Shales in the zone 

 of Climacog. Wilsoui, associated with Climacog. bicornis and Orthog. truncatus var. 

 intermedins, and also in the zone of Dicranog. Clingani, associated with Orthog. 

 truncatns. 



Collections. — British Museum (Natural History), Geological Survey of Scotland, 

 Sedgwick Museum, Lapworth, and the Authors. 



