422 Ml! JOHN S. FLETT ON 



to occur near Thurso, but not found as yet in the Rousay beds of Orkney. These 



are : — 



Homacanthus horcalis (Traq.). 

 Rhadinacanthus longispinus, (Ag.). 

 Mcsacanthus Peachi (Egert.). 

 Tltursius macrolcpidotus (Sedgw. and Murcli.). 

 Osteolepis microlepidotus (Pander.). 



Of these, the first is a rare fossil, and only described for the first time in 

 1892.* The second cannot be regarded as very abundant, seeing that the British 

 Museum Catalogue (1891) does not enumerate it as a Caithness species. The third has 

 not. so far, been mentioned in the literature of Orcadian geology, though Dr Traquair, 1 

 believe, has obtained a species of Mesacanthus from Orkney this last summer. That 

 these three rarities should be known from the carefully examined rocks around Thurso, 

 and not as yet from the Rousay beds of Orkney, to which attention has only lately been 

 directed, cannot be regarded as a strong argument against the theory that the one 

 series is the northern representative of the other. The two remaining fossils are of 

 more importance, seeing that they are regarded by Dr Teaquair as typical of the Thurso 

 rocks, and confined to them. One of these, Osteolepis microlepidotus (Pander.), is very 

 characteristic of them, and abundant in some of the beds ; but I have, at many different 

 times, examined collections of Orcadian fossils, and carefully searched the rocks for this 

 species, without ever obtaining a specimen which Dr Teaquair would admit belonged to 

 it. No doubt it has figured more than once in lists of fossils from Orkney, but the 

 identification is at present more than doubtful. It is possible that we have here a case 

 of local distribution the converse of that of Diplopterus, but at any rate the discrepancy 

 is one which cannot be overlooked, and it is to be hoped that further search in Orkney 

 will bring this fish to light. Thursius macrolepidotus (Ag.), it may also be anticipated, 

 will turn up in the Rousay beds, or at any rate its absence is not very remarkable when 

 we remember that only one satisfactory specimen of the other species of the same genus 

 has yet been discovered. Yet that, in that case, in the same quarry, two species which, 

 according to Dr Traquair, are typical of the Thurso rocks, should have been found 

 together for the first time in Orkney, is a surprising confirmation of the views he enun- 

 ciated in 1894, that they are type species of a special subdivision of the Orcadian Old 

 Red ; and that their distribution in Orkney, so far as yet known, is in complete accord- 

 ance with this supposition, has already been proved to be the case. They occur 

 always on practically the same horizon, and in the lowest beds they have never yet 

 been found. 



No other locality for these two fossils is at present known, and from the district in 

 which they have been longest and most thoroughly investigated they may be named 

 the Thurso Beds, or the Zone of Coccosteus minor (Hugh Miller) and Thursius 

 pkolidotus (Traq.). 



* Trans. Geol. Soe. Mdin., 1892. 



