Vol. 54.] 



FATJifA OF THE SKIDDAW SLATES. 



509 



from its apex, which here, as in other forms, must be regarded as 



the true virgula. The sicula is not 



placed symmetrically with regard to 



the stipes ; it is nearer the secondary 



stipe, i. e. the stipe with the second 



thecal series. 



The first theca (th. l^) appears to 

 originate from the sicula some dis- 

 tance above its aperture ; it grows at 

 first horizontally away from the 

 sicula, which is thus seen in obverse 

 aspect to be free on the left side near 

 the aperture. In the obverse view 

 theca 1^ is seen on the left side; 



Fig. 26. — Proximal end of 

 A, enlarged to show the 

 structure of the reverse side 

 of the rhahdosoma and 

 point of origin of th. 1^. 



origin 1 



Note change in cupvalure 

 of walls of (hecae 



after growing horizontally it curves 

 downwards. The connecting-canal 

 is narrow and oblique. In the earlier 

 part of its course theca 1^ is, in reverse 

 aspect, seen to be in close connexion 

 with the apertural part of the sicula. 



The walls of the earliest developed 

 thecae are concavely curved. They are 

 about 2^ inch (1*05 mm.) long ; those 

 developed later are longer. 



The stipes are narrowest at their 

 origin, but quickly attain their maxi- 

 mum width, 2^ inch (1*05 mm.), near 

 the bend. In the largest specimens 

 that I have seen the maximum width 

 was -V inch (1*58 mm.), but 4r: inch 



[X5.] 



Fig. 27. — More distal part of 

 stij^e of A, enlarged to show 

 the character of the thecal. 



[X5.] 



] 6 



20 



(1*05 mm.) is more usual. The thecae 

 of the initial part of the rhahdosoma 

 have their walls concavely curved, 

 but after about three thecae on the 

 primary stipe and two on the second- 

 ary, this concave curvature begins to 

 give place gradually to a slight convex 

 curvature. The thecae thus curve 

 away from each other, and the general 

 direction of the stipe is changed. 



The thecal walls in the distal por- 

 tion of the stipes are nearly straight ; 

 he thecae have a length of -^ inch 



of 

 the 



IK 



Fig. 28. — Enlargement 

 portion of B, showing 

 point of origin of th. 

 and the generally uyisym- 

 metrical appearance of the 

 stipes with regard to the 

 position of the sicula. 



[X5.] 



(1*58 mm.), and are 3 times as long as wide. 



Occurrence. — Middle Skiddaw Slates. 



Localities. — New Brow Quarry, Upper Lorton ; Dodd, Brack- 

 euthwaite ; Barf, near Keswick ; Brunstock Scar. 



