172 



BRITISH GRAPTOLITES. 



Polypary consisting of a fairly long biserial portion, 5 — 8 mm. in extent, and 



two imiserial stipes of a length of 7 — 8 cm. ; axillary angle 40° — 60°. Thecas 



twelve to nine in 10 mm., of the form of those of Dicellogra/ptus elegans, with 



curved free outer walls, and introverted and introtorted apertures which 



appear to fill more or less completely the ponch-shaped excavation that 



occupies rather more than one third the width of the stipe. 



Description. — The biserial portion of the polypary is composed of about 8 



thecse on each side; it widens conspicuously throughout its extent from 1 mm. to 



2*5 mm. ("8 mm. and 2 mm. when in relief), so that near the point of origin of the 



uniserial stipes it is fully two and a half times as wide as at its proximal extremity. 



The uniserial stipes are commonly straight, but sometimes exhibit slight curvature ; 



they are always long relatively to the biserial portion, and have an approximately 



uniform breadth of 1*3 mm. (1 mm. in relief). A few specimens have been found 



with a somewhat longer biserial portion (9 — 11 mm.), 

 and a more marked curvature of the stipes; but 

 at present it is doubtful if they are worthy of 

 varietal distinction (PI. XXV, figs. 1 g, h). 



The sicula is small, measuring only '6 mm. It 

 is seldom seen owing to the rarity of individuals 

 presenting the obverse aspect, but the stout aper- 

 tural spine may almost always be detected. From 

 specimens of D. Nicholson i preserved in relief, the 

 following structural details have been made out : 



Th. I 1 is small, and is developed from the left 

 side of the sicula (obverse view) ; it grows first 

 outward and upward for a short distance ; next it 

 bends round approximately at right angles, giving 

 off a spine at the bend ; and then growing vertically 



Figs 



108 a, b, and c. — Dicranogra ptus 

 Nicholsoni, Hopk. 



Biserial portion in full relief (reverse 

 view), showing septum, etc. On 

 same slab as PI. XXV, fig. 1 c. 



Biserial portion showing apex of 

 sicula (?) and mesial spines, etc. 

 Dobb's Linn, Lower Hartfell Shales. 

 Coll. Lap worth. 



Biserial portion (obverse view) show- 

 ing sicula. Ibid. Coll. Elles. 



upward opens within a small excavation. Th. I 2 



developed from the initial region of th. I 1 , and th. 2 1 

 from the initial region of th. 1 2 , while th. 2 2 appears to 

 develop from its apertural region. A septum is 

 produced between the two thecal series thus origi- 

 nated, and thereafter each theca develops from 

 the next proximal theca immediately below it, and grows at first straight upward, 

 then outward and upward, and finally upward again, forming the sigmoid curve 

 so characteristic of the thecal belonging to this genus and that of Dicellograjptus. 



The thecas gradually increase in size throughout the biserial portion, but never 

 attain their maximum length of 2 mm. until the two uniserial stipes have been 

 developed. They overlap for half their length. In well-preserved specimens the 

 thecae of the biserial portion of the polypary are seen to be furnished with stout 



