THE OLD RED SANDSTONE OF THE ORKNEYS. 421 



results of a comparison of lists of fossils from Caithness, Orkney, and Cromarty, and the 

 result is a division of the known fossils into three groups. One is that we have already 

 considered — the John o' Groats group. The second contains a series of fossils which 

 occur together only in the neighbourhood of Thurso. The list is as follows : — 



Eomacanthus borealis (Traq.). 



Rhadinacanthus longispinus (Ag.). 



Mesacanthus Peachi (Egert.). 



Cheir acanthus, sp. (perhaps 2 sp.). 



Coccosteus decipiens (Ag.). 



Coccosteus minor (H. Miller). 



Homosteus Milleri (Traq.). 



Dipterus valencienesii (Sedgw. and Murch.). 



Glyptolepis paucidens (Ag.). 



Thursius macrolepidotus (Sedgw. and Murch.). 



Thursius pholidotus (Traq.). 



Osteolepis microlepidotus (Pander.). 



(Scales, doubtfully resembling those of Gyroptychius). 



It will be observed that this list contains the type fossils of the Rousay series of 

 Orkney, Coccosteus minor (H. Miller) and Thursius pholidotus (Traq.); and when we 

 compare it with the list of the fossils I have found in those rocks, we find that the 

 following species occur in both : — 



Coccosteus minor (H. Miller). 



Thursius pholidotus (Traq.). 



Dipterus valencienesii (Sedgw. and Murch.). 



Glypiolepis paucidens (Ag.). 



Cheiracanthtis, sp. 



Coccosteus decipiens (Ag.). 



Homosteus Milleri (Traq.). 



With the exception of the first two, these are all contained in the list of fossils which 

 occur throughout the whole thickuess of the Orkney flagstones. In Orkney occurs one 

 species not yet found in Caithness, Asterolepis, sp. no v., which, considering that it is a 

 fossil of limited range, and confined to a few beds of rock, is an exception of no great 

 importance ; and two others present in Caithness, but not known from the vicinity of 

 Thurso, Osteolepis macrolepidotus (Ag.) and Diplopterus Agassizi. Of these, the latter 

 is one of the rarest of Caithness species, while in Orkney it is quite common, especially 

 in the quarries of Sandwick and Stromness. From the Rousay beds of Orkney I have 

 seen only one satisfactory specimen. It is probable that we have here a case of local 

 distribution, and that the absence of this fossil from the rocks around Thurso is due, not 

 to adverse conditions of preservation, but that rather it was from the first a species 

 characteristic of the more northern area, and hence more likely to persist there, and 

 occur on a higher horizon. On the other hand, we have a number of forms known 



