The Dipneusti and Crossopterygii 303 



characteristics are all well established in Lower Old Red 

 beds, the Osteolepids probably evolved in later Silurian 

 time from some ancestral type intermediate between prim- 

 itive dipnoan and holostean examples. But alongside these 

 the more specialized though related genera Glyptolepis and 

 Tristichopterus often occur, while all of them are accom- 

 panied by dipnoans like Coccosteus and Dipterus, as well 

 as Acanthodes and other elasmobranchs. 



As Geikie (2^:345), Flett (22:383), Goodchild 

 (77:591) and others have described, several of these in- 

 habited that area of Lower Old Red deposit which Geikie 

 named Lake Orcadie. But that the deposits formed in it 

 represented a long period of time, and varying organic 

 changes in the process, is shown by the beds being, in parts 

 of N. Scotland, from 5000 to 16,000 feet thick, while three 

 of the zones — the Stromness-Cromarty beds, the overlying 

 or Thurso-Rousay beds, and the highest or John-o-Groat 

 flags with Eday sandstones, — vary markedly in organic re- 

 lation. Thus Osteolepis macrolepidotus and Diplopteriis 

 agassizii are common to the two lower, but are absent 

 from the top zone. Thiirsius pholidotus is found in the 

 middle, but is absent in the lower and upper beds. Other 

 illustrative cases might easily be cited. 



As emphasizing the relation of primitive crossoptery- 

 gians to other groups, we give a list that is taken mainly 

 from Flett's papers. This also will bring out the environal 

 associations. 



Crossopterygians. 



Osteolepis macrolepidotus, O. microlepidotus, Thursius pholidotus, 

 T. macrolepidotus, Diplopterus agassizii, Glyptolepis paucidens, G. 

 leptopterus, Tristichopterus alatus. 



Other Associated Fishes. 



Coccosteus decipiens, C. minor, Dipterus valenciennesii, D. macropterus, 

 Homosteus milleri, Cheirolepis trailli, Acanthodes peachii, Diplacanthus 

 striatus, D. tenuistriatus, Cheiracanthus grandispinus, C. latus, C. 

 murchisoni, Gyroptychius angustus and G. macrolepidotus, Mesacanthus 

 pusillus, M. peachii, Homacanthus borealis, Palaeospondylus gunnii, 

 Rhadinacanthus longispinus, Microbrachium dicki, Pterichthys milleri, 

 P. productus. 



On p. 405 Traill remarks: "The calcareous and bitum- 

 inous flags are the chief receptacles of the fossil remains 



