444 



B HIT I S H G R A PT L I T K S . 





^ 



and more rigid, and having a proximal portion which is more robust to start 

 with, and widens more rapidly to the maximum breadth of 3 mm. (exclusive 

 of the spines). Sicula relatively small. Thecas eleven to seven in 

 10 mm., resembling those of M. Sedgwiclcii, but with less retroversion of 

 the apertural region of the theca?, especially distally, and with shorter 

 and stouter apertural spines. Overlap one-third to one-half of the thecal 

 length. 

 Description. — The polypary of M. Halli, like that of M. Sedgwiclcii, must often 

 have been of considerable extent, but is characteristically straighter and stiffer 



throughout, and the proximal region more robust, 



Figs. 305 a and b. — Monograptus Halli , . , , , , • i r • i 'Ji 



(Barrande). the maximum breadth being attained tairly rapidly. 



The thecsB of the proximal portion resemble in 

 general those of the corresponding portion of M. 

 Sedgwiclcii, but are somewhat larger and have more 

 conspicuous overlap; while the theca? of the distal 

 portion lose their retroverted character still more 

 quickly and more completely than the corresponding 

 theca? of M. Sedgwiclcii. Finally, the retroversion 

 becomes so slight that the aversion is the more 

 prominent characteristic, though in specimens 

 preserved in relief it is clear from the way in which 

 a , _ .,, the theca? grow that there is still a distinct tendency 



a. Proximal end of specimen, probably o 



referable to this species ; length of towards retroversion. The apertural spine is short 



sicula rather uncertain. W. side L 1 



of Upper Quarry, 550 yards E.S.E. am ] stiff. 

 of Faches-gau Farmhouse, Pont 



Erwyd, Cardiganshire. Geoi. Sur- From certain specimens it would seem that the 



vey of England and Wales. . . . . 



6. More distal portion, preserved as a toi'SlOU ot the thecal axis ill MOTWg. Halli may he 111 

 cast, Quarry S. side main road . 



from Devil's Bridge to Ehayader, an opposite direction (i.e. negative) to that which 



200 vards S. of Bodcoll Farm, - , ir o 7 • 7 •• j. / rn , 



Cardiganshire. Geoi. Survey of obtains as the rule in M. Sedgwiclcii, etc. (see Text- 

 Engiand and Wales. fi ^ .^ . &m} p] X LIV, fig. 8 d). The scarcity of 



specimens preserved in relief makes it impossible at present to determine if this 

 character is constant in this species and confined to it. 



In compressed examples of M. Halli, as in 71/. Sedgwiclcii, two extreme views 

 or appearances are presented to the observer as a natural consequence of the 

 torsion of the thecal axis apart from the direction of torsion. These may con- 

 veniently be distinguished as (1) a spinigerus appearance, and (2) a non-spinigerus 

 (or Sedgwiclcii) appearance, and these are connected by intermediate gradations. 



The extreme spinigerus appearance is that in which compression has taken 

 effect in the plane which includes both the terminal part of the thecal axis 

 and the spine itself. In this case the apertural margin of the theca is 

 typically shown as an approximately straight line, and the spine is displayed in its 

 real length. in the Sedgwiclcii or non-spinigerus appearance compression has 



