120 BRITISH CxRAPTOLITES. 



tertiary branches several centimetres in length, but often short compared 

 with the main stipes, and of uniform width throughout their length. 

 Sicula rarely visible. Thecas uniformly narrow, occupying about one third 

 the width of stipe, seven to eight in 10 mm., inclined at about 10°, eight 

 times as long as wide ; free two-thirds to one-half their length. Apertural 

 margins narrow, slightly concave when compressed, introverted. 



Description. — The polypary of this form must have attained an enormous size ; 

 a perfect specimen has never been seen by the Authors, but Carruthers states that 

 he has traced one for nearly 70 cm. (3 feet.) There is frequently a certain amount 

 of curvature in the main stipes near their proximal end, but the flexibility varies 

 greatly in different individuals. The secondary branches are given off quite 

 irregularly, and generally occur on one side only, but in some specimens they 

 appear on either side of the main stipe indiscriminately ; this, however, may be 

 merely due to the position in which the polypary came to rest, and therefore of no 

 specific importance. The earliest secondary branches may be given off quite close 

 to the sicula or at a distance comparatively remote from it. The secondary and 

 tertiary branches commonly show slight convex curvature. Fragments of them 

 may readily be distinguished from those of the main stipes by their uniform width, 

 fragments of the main stipes showing an increase in breadth for the greater part of 

 their length. The main stipes are exceedingly narrow at their origin, never, 

 apparently, exceeding *2 mm. in breadth; the maximum width observed on any 

 fragment is 1 mm., but in the specimens which attain the enormous length 

 mentioned by Carruthers it is possible that this dimension may be exceeded. The 

 width of each secondary or tertiary branch is the same as that of the parent stipe 

 at the point at which it is given off. 



The sicula is rarely visible, as in most of the complete specimens it gives rise to 

 a third stipe or branch. Siculos found in association with such forms probably belong 

 to the species, and are about 2 mm. in length. 



The thecal number seven to eight in 10 mm., the lower number being found near 

 the sicula, where the overlap is less than in the maturer parts of the stipes. Each 

 theca when fully developed has a length of 2 — 2'5 mm., is free for two thirds of its 

 length proximally, but subsequently overlaps approximately one half of its extent. 

 The thecal apertures usually occupy about one third of the total width of the stipe, 

 but near the sicula the apertures are so minute that they are hardly visible to the 

 naked eye, and the polypary frequently appears as a narrow unbroken line. 



Centribrachiate forms of P. linearis are of common occurrence. 



Horizon and Localities. — Hartfell Shales. 



8. Scotland : Dobb's Linn ; Hartfell Spa ; Mount Benger Burn ; Craigmichan 

 Scaurs; Rowantree Gutter; Thirlstane Scaur; Ettrick Bridge End, etc. 



Associates. — P. linearis occurs in abundance in one special zone in the Hartfell 

 Shales, where slabs of rock may be obtained entirely covered with fragments of it. 





