210 BRITISH GRAPTOLITES. 



CI. extremus is altogether a much smaller and narrower species. CI. Hughesi is the 

 same form as that described by Kurck as CI. undulatus; he noticed its similarity, 

 but thought it distinct. 



Horizon and Localities. — Birkhill Shales (zones of C. acuminatus, D. vesiculosus, 

 M. gregarius and C. cometa). 



8. Scotland : Dobb's Linn. Lake District ; Skelgill. Ireland : Coalpit Bay, 

 Donaghadee. Wales : River Twymyn, near Llanbrynmair. 



Associates, etc. — CI. Hughesi is a fairly common fossil in the Lower Birkhill 

 Shales. It occurs throughout the basal zones, but is most abundant in the 

 zones of M. gregarius and C. cometa, where it is found associated with M. gregarius, 

 M. cyphus, M. triangularis , G. cometa, M. lobiferus, etc. 



Collections. — Sedgwick Museum, British Museum (Natural History), Lapworth, 

 and the Authors. 



Climacograptus extremus, H. Lapworth. Plate XXVII, figs. 13 a, b. 



1900. Climacocjrajrfus extremus, H. Lapworth, Quart. Journ. Geol. Soc, vol. lvi, p. 134, figs. 22 a and 

 22 b a— e. 



Polypary from 0*5 to 1 cm. in length, and with an average breadth of - 5 mm., 

 with rounded proximal end. Sicula and virgula unknown, virgella short. 

 Thecse regularly alternate, twenty to fifteen in 10 mm. Apertural margins 

 small, introverted and introtorted, excavations occupying a small fraction 

 of the total width of the polypary. Septum undulating. 



Description. — The polypary may attain a length of over 1 cm., but smaller 

 specimens with a length of about 5 mm. are of 



Figs. 141 a— c — Climacograptus extre- L ° 



mus, h. Lapw. commoner occurrence. The free edges of the thecse 



may be vertical or strongly concave. 



The septal groove is a very characteristic feature ; 

 it runs from immediately below the aperture of one 

 theca to a corresponding position in the succeeding 



I 



theca of the opposite series, and is gently undulating 

 b or sharply angulated according to the variation in 



a. One of type specimens, incomplete. „ nmTlrplJ(! ; ni , 



Rhyd IHr Brook, Rhayader, Rhaya- CO" 1 ! 31 eSSlOn. 



der^Paie shales. Coll. h. Lap- Affinities.— CI. extremus agrees with Ol. Hughesi 



b. Complete specimen in relief, reverse j n ^q possession of an Undulating SeptlUU, and in 



view, showing form of thecse and r or' 



septum. Enlargement of pi. general characters of the thecse. From CI. Hughesi, 

 e. Fragment, reverse view. Same however, it mav be separated by (a) its smaller 



locality as Fig. 141 a. . J l . / n 



size, (b) the characters of the proximal end. 

 Horizon and Locality. — Turannon Shales (base of Rhayader Pale Shales). 



