280 BRITISH GRAPTOLITES. 



transverse concavo-convexity in specimens preserved in relief, are both very 

 characteristic. In general there is a diminution in breadth of outline as soon 



as the polyparv has attained its maximum width, 



Figs. I'M a M\i\b. -Pi'tahiijriiptun minor, . . . 



Kiies. but this is not invariably the case. Occasionally, 



| as a result of the extreme convexity of the reverse 



aspect, the distal end in that view has a pointed 

 ;./ appearance, but this is unusual. 



The sicula is slender and very loner, beina - 

 a. obverse aspect, showing apertural usually 3 mm. in length ; its apex usually lies on a 



part of sicula. Long Linn, Dobb's ' . . 



Linn; Birkhiii shales (zone of i. level with the aperture of th. 4 , but it tapers SO 



grcgarius). Coll. Elles. r> t • i t .!_• ji i ■ 



6. Reverse aspect, showing apical part hnely in an upward direction that m many speci- 

 mens it is hard to determine where the sicula ends 

 and the virgula begins. A virgella may often be detected. The sicula is 

 completely visible only when viewed in the obverse aspect, . and is free for one 

 third of its length on the right side. Th. I 1 originates close to the aperture of 

 the sicula, and grows at first in a direction parallel to the sicula itself, but 

 subsequently describes a further concave outward curve ; it is usually about 2 mm. 

 in length ; in the reverse aspect only its base and a small portion of the side are 

 visible, the rest is concealed by the initial growth of th. 1~, for in its earliest 

 stage th. 1- closely follows the direction of growth of the sicula. When the 

 polypary is shown in the obverse aspect th. I 3 appears to originate one third 

 the way up the sicula whereas in reality it has originated earlier; th. 1~ is 

 concavely curved like th. I 1 but to a rather less extent, and therefore the 

 aperture of th. 1" rises to a greater height than that of th. I 1 , though both 

 theca? have approximately the same length. The aperture of th. I 1 is at a level 

 of about 1*5 mm. above the sicular aperture, and lies at about 1*5 mm. from 

 it sideways. 



There are indications of the presence of a septum in the obverse aspect, but 

 none in the reverse, so that it must be incomplete (Tornquist). Examples 

 preserved in sub-relief show that it occasionally extends half way through the 

 polypary. 



The virgula is conspicuous and is often distally prolonged. 



Affinities. — P. minor may be confused at first sight with yonng forms oiPetalog. 

 palmeus s.s. and var. lulus, but it is distinguishable by the following characteristics : 

 (1) the sicula is longer and attains a greater height within the thecal series than 

 in Petalog. palmeus or in any of its varieties ; (2) the proximal end is always more 

 protracted; (8) the greatest width is reached later than in Petalog. palmeus or var. 

 hit us ; (-1) there is no trace of a septum in the reverse aspect as in Petalog. palmeus ; 

 (•")) in Petalog. minor the distal end is gently rounded off and the polypary increases 

 in width distally. 



Horizon and Localities. — Birkhiii Shales (zone of Monog. grcgarius). 



