DICELLOGRAPTUS. 



139 



Geoup [.—Type Vicellog. complanatus. 



Dicellogra/pti in which the thecse have straight ventral walls and horizontal 

 apertures. 



Dicellograptus complanatus, Lapworth. Plate XX, figs. 1 a — d. 



1880. Dicellograptus complanatus, Lapworth, Ann. Mag. Nat. Hist. [5], vol. v, p. 160, pi. v, figs. 

 17 a— e. 



b, and c. — Dicellograptus 

 nplanatus, Lapw. 



Stipes 8 cm. or more in length, generally straight, diverging at angles of 



270° — 240° from a conspicuous sicula. Virgella and lateral spines short and 



stout. Thecae ten to eight in 10 mm., free outer wall straight, overlapping 



one third to one half their length. Apertures horizontal and slightly intro- 



torted, opening within a shallow but well-marked excavation, which occupies 



about one third the width of the stipe and one fifth of the ventral wall. 



Description. — The stipes are usually straight, but occasionally they curve 



slightly outward throughout the whole of their 



extent in such a way as to increase the axillary 



angle. They are only "5 to "6 mm. wide at their 



origin, but the breadth increases somewhat rapidly 



up to 1 mm., and this width is maintained for 



the remainder of their length. 



The sicula has a length of about 1*5 mm. when 

 perfect, and is long and tapering, but it is so rarely 

 preserved complete that it has been described as 

 short and blunt. In the reverse aspect of the poly- 

 pary the sicula is almost entirely concealed by the 

 growth of the earliest thecae, and its presence and 

 position are only indicated by its apertural spine. 

 Few specimens are sufficiently well preserved to 

 show details regarding the structure and relations 

 of the thecae of the proximal end. Th. I 1 and th. I s 

 are small and grow almost entirely in a horizontal 

 direction, giving off a mesial ventral spine not far 

 below the apertures. Th. I 2 develops from th. I 1 in 

 the normal manner. Th. 2 1 is abnormally long for 

 a theca in the proximal region ; it arises from the basal part of th. I 2 and grows 

 horizontally for a considerable distance, crossing completely over the sicula. 



Reverse view, showing prominent 



virgella. Dobb's Linn, Hartfell 



Shales. Coll. Lapworth. 

 Young form, showing complete sicula. 



Dobb's Linn, Hartfell Shales. Coll. 



Wood. 

 Reverse view, low relief. Ibid. 



