TRTCH()(niAPTUS. 91 



This o-ouns mig'lit l)e rcgai-ded as iiitxM'iiiediak' in cliaractci' Ix'twccii tlic Diclio- 

 graptida^ and the liCptograptid*. It appvoaches closclv to Cit'iiof/niplii.s (of 

 Hall) in the teiinity (if its stipes and general mode of bi-anching. its characters 

 in many respects a])proximate also to those of Pferograpfus, TTolm ; Ijnt the 

 angle of divergence of the two main stipes, which in that genus is less than 

 90°, is here over 1 80°. The form of the thecse, however, so far as can be made 

 out, appears to link it rather with the Dichograptidoe than the Leptograptid.B. 

 Hence it seems best, in the present state of our knowledge, to i^egard it theoretically 

 as a very slender reclined compound DicJi/mngnipfin^, from the tli(>cal margins of 

 which simple lateral bi-anches are given off. 



Only one British species has yet been recorded, viz. Tricho/jrajifns fnigilis. 



Trichograptus fragilis (Nicholson). Plate XII, fig. 5. 



18G9. Biclwgrapsus fragilis, Nicholson, Aun. Mag. Nat. Hist. [4], vol. iv, p. 232, pi. xi, figs. 1 — 3. 

 187G. Trichograjdiis fragilis, Nicliolson, Geol. Mag., doc. 2, vol. iii, p. 248. 

 1898. Trichograptus fragilis, Elles, Quart. Journ. Geol. Soc, vol. liv, p. 476. 



Main stipes straight, 1 cm. or more in length, originating from a conspicuous 

 sicula, and converging iipward and backward upon the nema ; from their 

 thecal margins three (or more) lateral branches are given off near the 

 proximal end. Thecce nine to ten in 10 mm., inclined at 28°, average length 

 1'5 nun., three to four times as long as wide; outer walls straight, over- 

 lapping one quarter to one third their length. Apertural margins normal, 

 straight. 

 Description. — The whole polypary is very small ; the main stipes seem rather 

 straighter than the lateral branches, which have a length of about 2 cm. All the 

 stipes are characteristically slender and thread-like, having an average width of 

 •5 mm. The first lateral stipe is given off at a distance of about 1 mm. from the 

 sicula, and there is a distance of about 1 "4 mm. between each of the others. This 

 first seems to originate at about the length of a single theca from the sicula. 



The sicula is small, measuring only '5 mm. in 



Fig. .")3. — Trichograptus fragilis, i,i ,t t • i • • "xj^i 



Nicliolson. ■ length ; nevertheless it is conspicuous, owing to the 



V j extreme tenuity of the stipes. 



■j^ Affinities. — T. fragilis may be distinguished from 



^^j all other Dichograptidae by its extreme tenuity and 



\ its mode of growth. 



v^ Horizon and Localities. — Llanvirn, Upper Skid- 



>^j daw Slates. 

 V) Lahi District : Thornship Beck, near Shap ; 



Thecce on one of the lateral branches. Ashlock Sikc. 

 Lnlargeuient of PL XII, fig. 5. 



Associates, etc. — The associates of T. fragilis 



arc 



