188 BRITISH GRAPTOLITES. 



visible. The virgella is always present and is short, straight, and robust, 

 measuring about 1 mm. in length. 



Theca l 1 originates at a point about 1 mm. above the aperture of the 

 sicula; it grows downward on the side of the virgella some little distance 



below the aperture, and then turns back, growing 



Fin. 121 a and b. — Climacograptus , . 



rcctanguiaris (M'Coy). upward and slightly outward, so that its aperture 



comes to lie on a level Avith or slightly above its 

 point of origin. Th. 1~ grows obliquely across the 

 back of the sicula, leaving it free on one side for a 

 small fraction of its length. 



The septum originates at once between the 



apertures of th. I 1 and th. I 2 , or at latest on the level 



of the aperture of th. I 2 . The virgula is often 



a b prolonged distally for a considerable distance. 



"' ^S^men/o^art'o^n^xxvJ Affinities.— There appears to have been consider- 



fig. 5a. a )jj misapprehension respecting this species of 



b. Youno- specimen probably referable L l . . 



to this species, obverse view show- M'CoV ', and ( 7 iinacoqra pi IIS TomquistL which IS ill 

 mg sicula Twymyn River, near ,; J J s 



Lianbrynmair, Doigadfan (Birthiii reality very different, has been commonly referred 



Beds). Coll. Elles. '[ J . mi , -, . . , 



to this species. The above description has been 

 drawn up from M'Coy's type specimens which are in the Sedgwick Museum ; and 

 his species appears therefore to have the following characteristic features : 



(1) Gradual increase in breadth over first 1*5 cm. till 2"5 mm. is attained. 



(2) Short conspicuous virgella proceeding from a sicula, which is free for 

 nearly half of its length on one side. 



(•'!) Complete septum. 



It is therefore closely related to CI. scalaris (with which its theca? agree in all 

 essentials), but differs from it in size and in the greater breadth, attained by gradual 

 and persistent widening. From CI. medius it differs in having a shorter virgella, 

 more gradual widening, and more complete septum; from 01. Tornquisti, with 

 which it appears very frequently to have been confused, it differs entirely in the 

 character of the proximal end. 



Horizon <<ml Localities. — Lower Birkhill Shales and equivalents. 



S. Scotland : Dobb's Linn ; Garple Linn ; Belcraig Burn ; Crosscleuch ; Ritton- 

 side; Penwhapple Glen, Saugh Hill, etc. Lake District: Skelgill, etc. Wales: 

 Pary's Mountain ; Llanystwmdwy. Ireland: Pomeroy ; Donaghadee. 



Associates, etc. — GUmacog. rectangularis is a common fossil in the Lower 

 Birkhill Shales (particularly in the zones of Dialog, vesiculosus and M. gregarius) i 

 and their equivalents in the British Isles. Its associates are the entire Lower 

 Birkhill fauna. 



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

 Kearnsides, and the Authors. 



