H. 0. Nicholson— Graptolites in the Skiddaw Slates. 343 



Slates, 1 but this figure was not accompanied by any description, and, 

 so far as I am aware, no descriptiou has beeu published by any 

 subsequent writer. 



Nevertheless, the form of the polypary in this species is so 

 characteristic, as will be seen from the subjoined figure (Fig. 4), 

 that the species has been accepted as valid by Professor Lapworth 

 in his classical paper on the geological distribution of the lihabdo- 

 phora, 2 as also by Herrmann, Tullberg, and others. 



Fig. 4. — Lidymograptus v-fracius, Salter. Natural size. 

 Ordovician (Skiddaw Slates). 



In the original Didymograptiis Y-fractus, Salter, the basal angle 

 of the two branches of the polypary is considerably more open than 

 in the specimen here under consideration, and the curvature of the 

 branches is effected in a gradual upward sweep. On the other hand, 

 in my specimen, the basal portions of the branches diverge very 

 slightly, and then are reflected abruptly, at right angles to a line 

 traversing the axis of the sicula. 



The distinction just pointed out is so marked, that Professor Lap- 

 worth, to whom I submitted the specimen, suggested, that though it 

 might be placed under Didymograptiis y-fractus, it would be well to 

 distinguish it by a varietal name; and following his advice I have 

 named the form Didymograptiis y-fractus, Salter, var. volucer. It is 

 by no means impossible that future discoveries may serve to raise 

 this to the rank of a distinct species. 



With the exception of the doubtful form recorded by Baily from 

 the Lower Bala rocks of Ireland, under the name of Didymograptiis 

 Hisingeri, 6 all the species of the group Didymograptiis y-fractus are 

 found in the Arenig Beds. Two species belonging to this group 

 have been described by Tullberg, 4 from the Arenig Beds of Sweden, 

 under the names of Didymograptiis balticus, and Didymograptiis 

 vacillans, but it is quite unnecessary to discuss the characters of 

 these, as they show no marked resemblance to the specimen now in 

 question. Salter, in the paper above quoted, refers to a Graptolite 

 recorded by McCoy from the Ordovician rocks of Victoria, under 

 the name of Didymograptiis Pautonii, and states that this shows 

 a considerable similarity to his Didymograptiis y-fractus. McCoy's 

 form, at the time when Salter wrote, had neither been described nor 

 figured, but the resemblance suggested by him was accepted by 

 subsequent writers. Hence Mr. R. Etheridge, jun., in 1874, 

 described and figured a Didymograptiis from the Ordovician rocks of 



1 Quart. Journ. Geol. Soc. vol. xix. p. 140, fig. 13e. 



2 Ami. and Mag. N. Hist. ser. 5, vol. iii. 



3 I regret that I am unable to find a reference for this. 

 * Geol. For. Fork. No. 58, Bd. v. No. 2, p. 39, 1880. 



