286 BRITISH GRAPTOLITES. 



width for a short distance, and narrowing thence in a distal direction. 



Sicula small. Thecal always few in number, never exceeding twelve, with 



all the apertural margins situated close to the distal extremity and forming 



a kind of crown ; very long, straight, narrow, and slightly curved tubes, 



inclined 5° — 10°; apertural margins normal. 



Description. — The sicula is completely visible (Tornquist) in both obverse and 



reverse aspects of the polypary. It is very small compared Avith the great length 



attained by the earliest thecee, measuring only 1*5 mm. ; when viewed in either 



aspect it is seen to be free for its entire length (as in all forms of the Monograptidos). 



via. 197.- cephaiograptu, comda Tn - I 1 originates close to the aperture of the sicula, 



and grows upward, making a very gentle outward 

 curve. Th. Y 2 does not develop from th. I 1 till the 

 latter has grown some considerable distance bej^ond 

 the sicula, so that the latter is completely visible in 

 both aspects of the polypary. 

 Reverse aspect, showing sicula. Pary's The virgula is seen to arise, as usual, from the 



Mountain, Anglesea ; Llandovery ° 



Beds. Coll. g. j. Williams. apex of the sicula, but at first it runs along the dorsal 



side of th. I 1 , and appears subsequently to pass into the centre of the polypary. 

 The virgular tube is often distally prolonged and not infrequently split at some 

 point along its length. 



The entire proximal end of the polypary is slender and is greatly protracted, 

 the attenuation being especially marked at a short distance from the proximal 

 extremity, at which point the polypary is frequently broken off. When the poly- 

 pary is complete a slight swelling is observable at its extreme proximal end, 

 indicating the position of the aperture of the sicula. The proximal portion of the 

 polypary appears to have been somewhat flexible, for most compressed examples 

 have a somewhat sinuous outline. The two thecaa earliest developed are very long 

 and narrow ; th. I 1 may be as much as 3 cm. or more in length. All the thecce 

 subsequently developed are shorter than these. 



Polyparies from different localities are apt to vary slightly in width, apart from 

 considerations of preservation. The compressed specimens commonly found in 

 Britain have a maximum width of about 2'5 mm. (usually attained opposite the 

 aperture of th. I 1 ), and this may be maintained up to the level of the aperture of 

 th. I- before decreasing ; or a decrease in width may take place before the level of 

 the aperture of th. l~ is reached. The thecas are often somewhat curved, with 

 the exception of those developed last, which are short and straight. Growth -lines 

 may often be detected on specimens preserved in relief. 



The septum is invisible (Tornquist) in the reverse aspect of the polypary ; it is, 

 in fact, so far reduced that it can scarcely be said to be more than a mere fold of 

 the periderm on the obverse aspect. This median fold is not continued to the 

 proximal end of the polypary. 



