Vol. 54.] FAUNA OF THE SKIDDAW SI,ATES,. .491 



Occurrence. — Middle Skiddaw Slates. 



Localities. — The species seems very abundant in the Skiddaw 

 Slates ; it is recorded from Eaudal Crag, Skiddaw ; Bassenthwaite 

 Sand-beds ; Outerside ; White House Pell, Skiddaw ; east of Dodd 

 "Wood, Keswick ; JBarf, Frozen Gill ; Gibraltar, Skiddaw. 



(c^) Tetragkaptus pendens, sp. nov. 



* Ehabdosoma small and very slender, consisting of two pairs of 

 descending stipes, which after the first divergence are directed 

 vertically downwards, and run nearly parallel to each other. The 

 sicula is conspicuous and fairly long, being -^^ inch in length 

 (1-58 mm.). The stipes are narrow and of uniform width through- 

 out their length, never exceeding ^^^ inch ('o mm.); the inner 

 margin is celluliferous. The cells are long narrow tubes of uniform 

 width, and number between twenty and twenty-four in the space 

 of an inch (eight to nine in 10 mm.) ; they are inclined 15° to 20° 

 to the general direction of the stipe ; their outer wall is straight, or 

 may have a slight concave curvature near the proximal end. The 

 cells are in contact with each other for rather less than half their 

 length, which is commonly J^inch 



(2-1 mm.). Apertures straight, ^ig. 13.— Tetragraptus pendens, 

 occupying more than half the g .^^^^ f-^^i^ Postlethiuaite). 

 total width of the rhabdosoma. 

 Apertural angle about 120°.' 



This delicate little species is 

 related to Tetragraptus fruticosus 

 (Hall) by its general form, but 

 may be readily distinguished from 

 that species : — 



(1) It is much more slender, 

 and never seems to exceed -| inch [Nat. size.] 

 in length. 



(2) Its stipes are parallel and of uniform width, while those of 

 T. fruticosus are divergent and increase gradually in width 

 throughout their length. 



(3) There are more cells in a given unit of length, twenty to 

 twentj^-four compared with fifteen. 



(4) The cells are inclined at a very much lower angle, 15° 

 instead of 38°. 



From all other known species of Tetragraptus the present species 

 is distinguished by its form . 



Structure. — The development of the rhabdosoma appears to 

 commence precisely in the way that Holm has described as charac- 

 teristic of the genus Didymograptus,^ but after the develop- 

 ment of one cell each stipe undergoes dichotomous division, resulting 

 in the production of two pairs of stipes, which appear to be similar 

 in every respect. 



Occurrence. — Middle Skiddaw Slates. 



One specimen of this species occurs on a slab with Azygogra^jtvs 

 suecicus^ Moberg, another on a slab with Didgmograptus V-fractus, 



^ Op. Jam cit. Geol. Foren. Stockholm Forhandl. vol. xvii (1895) p. 319. 



