TETRAGRAPTLTS. 63 



Tetrar/nijdus serra and Didi/mograjifiis civ. fiMfonnis. It is now, however, known 

 to occur in the Lake District. 



Group III. — Type T. pendens. 



Tetragrapti in which the stipes are directed subvertically downward and are of 

 uniform widtli tliroughout their length ; the thecse are closely set, and are in 

 contact for about one half their extent. 



Tetragraptus pendens, Elles. Plate VI, figs. 3 a—d. 



1898. Tetragraphis i^endens, Elles, Quart. Jouru. Geol. Soc, vol. liv, p. 491, fig. 13. 



Main stipes short, from 12 to 18 mm. in length, directed subvertically downward, 



ultimately running nearly parallel to each other, and having a uniformly 



slender Avidtli of "G mm. throughout their length. Sicula conspicuous. ThecEe 



nine to twelve in 10 mm., inclined 15° — 20°, about three times as long as 



wide, free one third to half their length. Apertural margins normal, 



straight, occupying more than half the total widtli of the polypary. 



Description. — The whole polypary is small, the stipes not exceeding 18 mm. in 



length; they are also more or less uniformly narrow ('6 mm.), and their tliecae are 



always borne on the inner margins. 



The sicula is conspicuous; it measures about 

 Ym.zs.-Tctracirapius pendens, ^w^s. |.(; ^^^^^^^^ .^^^j ^^xQ stipcs Originate in a manner pre- 

 cisely similar to tliat observed in the Didymograpti 

 of the dependent series, but after the development of 

 one tlieca on each side of the sicula each stipe 

 imdergoes dichotomous division, resulting in the 

 production of two pairs of stipes which are similar 

 in every respect. 



Proxiuial en,I, reverse view, preserved '^^'^^ ^llCCOS are loug, UarrOW tubcS of Uniform 



asanmjpr.-ssp. Euiargement of ^yidth ; they luivo an average length of about 

 I'G mm. ; their outer walls are straight, or show 

 slight concave curvature near the proximal end. 



Affinities. — This delicate little species somewhat resembles T. fruticosus in its 

 general mode of growth, but may readily be distinguished from that species since — 



(1) it is smaller and far more slender, 



(2) its stipes are parallel and of uniform width, while those of T. fniticusus 

 are divergent distally, and increase in width throughout, 



9 



