412 PROFESSOR GEIKIE ON THE 
fossil evidence the conclusion, otherwise reached, that the Orkney beds belong 
to the upper part of the Caithness series. Of the other crustacean, so far as I 
have been able to learn, only one specimen has been recognised. This is in 
the British Museum. With the view.of obtaining its history and zoological 
relations, I applied to my friend Mr Henry Woopwarp, who kindly supplied me 
with the following notes :—“ The slab came from near Stromness [it exactly 
agrees with the well-known Skail flagstone so crowded with fossil fishes], and 
was purchased some years.since of Mr J. R. Grecory, together with specimens 
of Pterichthys cancriformis, Cheirolepis Trail, Cheiracanthus minor, C. pul-— 
verulentus, Diplacanthus crassispinus, Osteolepis brevis, Gyroptychius angustus, 
&c, All these fishes are in the same dark, highly bituminous shale, and they 
are preserved as bright jet-black enamelled objects. I have never doubted the 
fossil on this slab being Pterygotus ; but it is at the same time very obscure and 
fragmentary. It is part of a basal joint of one of the large ectognaths or swim- 
ming jaw-feet (maxillipeds), and has traces of the characteristic sguame along 
the lower border. The fossil is much distorted in form.” 
The genus Pterygotus has always been regarded as specially characteristic 
of the higher part of the Upper Silurian and lower part of the Old Red Sand- 
stone system. Its occurrence high up in the flagstone series of Orkney is sig- 
nificant of the true position of that series. Taken in conjunction with the other 
evidence already cited, it helps to indicate that the Old Red- Sandstone of. 
the north of Scotland was not so entirely posterior to that of Forfarshire as to 
deserve the name of a “ middle” series, but rather that both may have been 
deposited during the life of the same crustacean forms. 
The Orkney ichthyolites have been described chiefly by Acassiz, partly by 
M‘Coy. But they have never had the advantage of being collected by a resi- 
dent, keen-eyed, enthusiastic, and able naturalist like Mr Praca. No precise 
record appears to have been kept of the localities from which they have been 
obtained. In the fossil lists the word ‘‘ Orkney” is usually the only indication 
of the source of the specimens. It is as yet impossible, therefore, to construct 
any table of ranges for the Orkney fishes. If, however, the Orkney flagstones 
are the northern extension of the upper rather than the lower groups of Caith- 
ness, we may expect their ichthyolites to bear out this correlation. The list of 
species given in Table IT. shows it to be thus sustained. It may be surmised that 
the species at present peculiar to Orkney will probably in great measure prove 
to be identical with forms from Caithness and the Moray Firth. Very consider- 
able differences of aspect arise from the peculiar conditions of fossilisation. 
An Osteolepis, for example, from the Orkney flagstones, with its pitch-black 
glossy surface, and its frequently indistinct preservation of scales and fins, pre- 
sents a very distinct appearance from the dull black-grey hue and admirably 
defined form and sculpture of a Caithness specimen. Both of these, agai, 

