I20 Evolution and Distribution of Fishes 



list of those recorded: Tristicoptenis alatus^ Diptenis ma- 

 cropterus and D. valenciennesii (Fig. 9c), Microbrachius 

 dicki, Homosteiis milleri, Coccosteiis decipiens and C. 

 minor, Diplacanthus striatus, and D. tennhtriatiis, Mesa- 

 canthus peachi, Cheiracanthiis murchisonii and C. latiis, 

 Glyptolepis paucidens, Gyroptychius microlepidotus, 

 Osteolepis microlepidotus (Fig. 9b), and macrolepidottis, 

 Thursiiis macrolepidotiis and T. pholidotus, Diplopterus 

 agassizii, Mesacanthus pusilliis, Pterichthys milleri, P. 

 productus, and P. oblongus, Cheirolepis trailli, Glyptopty- 

 chius angiistus and Parexus sp. These deposits therefore 

 are rich alike in individuals and species. 



While the remains of these fishes are often scant or 

 absent from the coarser sandstones or shales, they may 

 be crowded in countless numbers in the harder shales and 

 flagstones. Flett says (22:383) that these "calcareous 

 and bituminous flags -are the chief receptacles of the fossil 

 remains enclosed in these rocks. The fossil collector very 

 soon learns that the best specimens are obtained in a brittle 

 hard, usually slaty and thin-bedded rock, which rings to 

 the hammer like a piece of metal. This is in some measure 

 due to the compactness and impermeability which is con- 

 ferred on these rocks by their abundant calcareous matter." 

 Though this type of rock might quite be a fine argillaceous 

 lacustrine or fluviatile deposit, that afterwards became 

 baked, the writer cannot but suggest that such may rather 

 have been due to deposit of volcanic dust that suddenly 

 killed, covered up, and permeated the armor of each in- 

 dividual. Favoring this is Flett's observation: "it would 

 seem as if these species had been unsuited to the new 

 environment in some manner or other, and their extinction 

 had been rapid and complete. The flags so crowded with 

 fossil remains of Diptenis valenciennesii — only a few of 

 which have attained their full size — irresistibly impress on 

 the mind the idea of a sudden extermination." Other 

 writers express a like opinion. 



The unconformability noted by Geikie and his succes- 

 sors, between deposits of the Lower and Upper Old Red 

 rocks, must represent a considerable lapse in time. But 

 before treating of this it may be opportune to observe, 



