194 



UKITlSIl GRAPTOLITES. 



'), c, ami il . ( 'I iiii<ico<irnji!u.* 

 bin, mis (Hall). 









maximum width of 2"5 nun. is attained, then 

 continuing with parallel margins up to the 

 distal end; proximal end provided with two 

 conspicuous spines. Sicula small. Thecae 

 twelve to ten in 10 mm., having an average 

 length of 1'5 mm. and overlapping about one- 

 third of their extent; apertural margins 

 horizontal, situated in elliptical excavations 

 occupying one-third of the ventral margin 

 and one-quarter the width of the polypary. 

 Description. — The polypary varies very much in 

 size and may attain a length of 6 cm. or more, 

 but specimens having a length of 3 — 4 cm. are of 

 commoner occurrence ; when compressed the poly- 

 pary measures "8 mm. — 1 mm. in breadth at its 

 origin, but specimens preserved in relief generally 

 only measure "6 mm. The increase is gradual in a 

 distal direction, but the maximum breadth is usually 

 attained within the first four centimetres of length. 

 At 1 cm. the polypary is 1*5 mm. wide, at 2 cm. 

 2 mm., at 3 cm. about 2*2 mm., and at 4 cm. 2*5 mm. 

 The sicula is small ; when complete it measures 

 approximately 1 mm. in length, but since the whole 

 of the apical region is imbedded in the fully 

 developed polypary the visible length as a rule is 

 only '5 mm. In the case of very young specimens 

 the sicula is completely visible. The virgella can 

 often be seen as a small stout projection from the 

 sicular aperture, and the virgula is occasionally prolonged beyond the distal 

 extremity of the polypary. 



Th. I 1 originates about "5 mm. above the aperture of the sicula (i.e. at the top 

 of the visible portion in a mature specimen) and grows downward until it reaches 

 the level of the aperture. It then bends round and grows outward and slightly 

 upward, so that its own aperture is slightly below the level of its point of origin. 

 A spine is always present on each side of the base of the polypary; in some cases 

 these spines arc merely outgrowths from the outer walls of the earliest thecas just 

 below their apertures, but often a complete modification of th. I 1 and th. 1" to spines 

 appears to have taken place and no aperture can be discerned. There appears to 

 lie every stage between these two extremes. The spines vary very much in size, 

 and, ;is Nicholson long ago observed, they may be anything from 1 mm. to 1 cm. 

 in length, and may be curved and long, or straight and short; the angle 



a. Proximal end showing part of sicula, 



and apertural edges of proximal 

 theca: prolonged into spines. Dobb's 

 Linn, Lower Hartfell Shales (zone 

 of Climacog. Wilsoni). Coll. Elles. 



b. Proximal end showing aperture of 



th. I 1 . Ibid. 



c. Distal thecae, half profile view. Dobb's 



Linn, Lower Hartfell Shales. Coll. 

 Wood. 



d. Distal thecee, sub-scalariform view. 



Ibid. Coll. Lapworth. 



