Vol. 54.] 



FAUNA OF THE SKIDD AW SLATES. 



485 



they fchen divide 

 dichotomously, 

 giving rise to the 

 eight stipes of the 

 3rd order, which 

 make with each 

 other an angle 

 of about 70^^ 

 These stipes may 

 be as much as 3 

 inches in length. 



The whole rhab- 

 dosoma is very 

 slender, never at- 

 taining a greater 

 width than X- 

 inch ('5 mm.). 



The thecae are 

 not very clearly 

 seen, but they 

 seem to be closely 

 set, and number 

 thirty-two to the 

 inch (thirteen in 

 10 mm.). 



Locality. — 

 Outerside. 



Pig. 10. — Dichograptus separatus, 

 (coll. Woodwardian Mus.). 



sp. nou. 



[Nat. size.] 



Genus Tetragraptus, Salt. 



(/) Tetragraptfs quadribrachiattjs (Hall). 



1858. Graptolithus quadrihrachiafus, Hall, Greol. Surv. Canada Rep. 1857, p. 125. 



1863. Tetragrapsus crucialis, Salt., Quart. Journ. Geol. Soc. vol. xix, p. 137, 

 fig. 8 6. 



1865. Graptolithus quadrihrachiafus, Hall, ' Grapt. of the Quebec Group,' Geol. 

 Surv. Canada, dec. 2, p. 91, pi. v, figs. 1-5, & pi. vi, figs. 5, 6. 



1868. Tetraqrapsus quadribrachiatus, Nich. Quart. Journ. Geol. Soc. vol. xxiv, 

 p. 131. 



1875. Tetragraptus quadribrachiatus, Hopk. & Lapw. ibid. vol. xxxi, p. 649 & 

 pi. xxxiii, figs. 9 a &b. 



Tetragraptus quadribrachiatus (Hall) seems to be abundant in 

 the Skiddaw Slates. There are specimens of all ages and sizes, but 

 only a few are sufficiently well preserved to show the characters of 

 the thecse ; the rest merely show the typical form of the species. 



The ultimate stipes are generally straight and rigid ; they are 

 slender at their origin, and gradually increase up to a maximum 

 width of y^2- i^c^ (2'1 mm.) in the largest specimens, but often only 

 -Jg- to yig- inch (1"2 to 1*5 mm.) in the forms that are of common 

 occurrence. 



^ The above-mentioned angles of divergence refer only to one aspect of the 

 rhabdosoma ; this was the same in' all the specimens known to me, and the 

 angle was therefore constant. 



