Vol. 54.] 



rATJNA OP THE SKIDDAW SLATES. 



499 



(B) Ee verse 8ide. — This specimen is preserved in relief, and is 

 I inch (6*3 mm.) long. 



In this view of the rhabdosoma the greater part of the sicula and 

 the first theca are hidden by the growth of th. 1^ and th. 2\ so that 

 there are only visible the apex of the sicula with the filamentous 

 virgula and the initial part of theca 1\ and, at the base, the apertures 

 of the sicula and theca 1^ 



The connecting-canal between theca 1^ and theca V is clearly 

 seen, just below the apex of the sicula and rather above the level of 

 the initial parts of the thecae which have developed later. There 

 appears also to be a connecting-canal (or something of an homo- 

 logous nature) between 



theca 2^ and theca 1". Fig. 18.— Didymograptus gibberulus, 

 This would seem to Mch. ; reverse asj^eet,' coll. Wood- 



indicate that theca 2^ ivardian Museum. 



develops from theca 1-, 

 instead of developing 

 normally from theca V. 



Unfortunately, I am 

 unable to say for certain 

 whether such is always 

 the case in this species or 

 not, for this is the only 

 specimen that I have seen 

 of this aspect of the rhab- 

 dosoma, sufficiently well 

 preserved to show these 

 details. It may be acci- 

 dental. Development 

 appears, at any rate, to «!= apex of sicula. 

 have proceeded normally a^=aperture of sicula. 

 after the growth of the &^=initial^ part of 



earliest thecse, each theca ,0 ^^^^ /- • 



„ , . ', , . /r = aperture or theca 1 \ 



01 each series developing ^ =viro'ula. 



: theca 1^. 



V- 

 [X5.] 

 [X5.] 

 [From White House Fell, Skiddaw.] 



e^ = connecting - canal 

 between thecse 1^ & 



e2— connecting-canal (?) 

 between thecte P & 



/= theca 2\ 



from the theca next to it d 



on the side nearest the 



sicula. Growth-lines may be detected in places. 



In other particulars the structure of the rhabdosoma seems to be 

 identical in both aspects. 



Occurrence. — On slabs with Didymograptus patulus (Hall), 

 Azygograptus suecicus, Moberg, and Tetragraptus quadribrachiatus 

 (Hall), in Middle Skiddaw Slates. 



Localities. — Randal Crag and White House Fell, Skiddaw; 

 Bassenthwaite Common. 



(Jc) DiDTMOGEAPTUs NiTiDus (Hall). (Figs. 19 & 20, pp. 500, 501.) 



1858. Graptolithus nitidus, Hall, Gaol. Surv. Canada Rep. 1857, p. 129. 



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

 Canada, dec. 2, p. 69 & pi. i, figs. 1-9. 



1868. Didyviograpsus nitidus, Nich. Quart. Jouru. Geol. Soc. vol. xxiv, p. 135. 



1874. Didymograptus nitidus, Etheridge Jun., Ann. Mag. Nat. Hist. ser. 4, 

 vol. xiv, p. 6 & pi. iii, fig. 20. 



A species which resembles Hall's B. nitidus more closely than any 



