64 BRITISH GRAPTOLITES. 



(o) tlicro arc more tlicca3 in a given unit of length, nine to twelve, as compared 

 with seven, in 10 mm., 



(4) the thecge are inclined at a much lower angle, 15° as compared with 35°. 



From all other known Tetragrapti it is distinguished by its form. 



Horizon and Locality. — Arenig, Middle Skiddaw Slates. 



Lake District. — Bai'f, near Keswick. 



Associates, etc. — T. '])endens occurs in the Skiddaw Slates, associated with 

 Azygog. suecicus, Didymog. deflexus, D. gibberulus, and Phyllog. cfr. typus. It has 

 not as yet been recognised elsewhere. 



Tetragraptus Postlethwaitii, Elles. Plate VI, figs. 1 a, b. 



1898. TetragraiJtus Postlethwaitii, Elles, Quart. Jouru. Geol Soc, vol. liv, p. 492, fig. 14. 



Main stipes 2*5 — 9 cm. in length; slightly curved, 'directed more or less verti- 

 cally downward, and having a uniform width of I'G mm. Sicula conspicuous. 

 Thecaj eleven to twelve in 10 mm., inclined 30°, three times as long as wide, 

 in contact half to two thirds their length. Apertural margins normal, 

 straight. 

 Description. — Both stipes of the first order are very short, but the main stipes 

 often attain a considerable length. One specimen showed stipes with a length 

 9 cm., but the width never exceeded 1*6 mm. 



The sicula has a length of about 2 mm. ; the 



Fio. 39.— Tc^i'ajympiws Postlethwaitii, . . . •, ^ '^ ■ t 



Elles. crossing canal is nearly horizontal. 



1^ The tlieca3 are 2 mm. in length ; they widen 



,s{^^':^!^"\:,^ slightly towards their apertures ; their outer walls 



h^K ^^*6 curved, and the a])ertural angle is commonly 



„-. V'^^*^ 130° after compression. 

 ^■-'■i Affinities. — T. Postlethwaitii resembles T. friiti- 



rroxi.naWnul.jevei^e view^ Enlaryc- ^^^^^^ ^^^^ rp ^^g,, J^.„^, Jj^ Jtg Uiodc of growtll, but 



differs from them in some important characters. It 

 may bo distinguished from T. fruticosus — 



(1) by the absence of widening in the stipes, 



(2) in the lower angle of inclination of the tlieca), and the greater number in a 

 given unit of length. 



It differs from T. p)endens — 



(1) in its larger size, 



(2) in the higher angle of inclination of the tlieca3, and the greater number in 

 a given unit of length. 



Horizon and LorMlities.—Avomg, Middle Skiddaw Slates. 

 LaJce District : CarLside Edge ; Barf, near Keswick. 





