TEIASSIC FAUKA. 157 



in the Pennian. These ancient deposits have as yet yielded no 

 traces of either birds or mammals. 



Tliassic Eauna, — In the first of the Mesozoic series of forma- 

 tions, the Triassic, we enter, as it were, upon an entirely new phase 

 of organic development. Many of the more characteristic groups 

 of organisms of the preceding era have now either completely dis- 

 appeared, or only survived in such diminished numbers as to con- 

 stitute but a very insignificant element in the new fauna. Thus, of 

 the class Brachiopoda but comparatively few of the older generic 

 types are represented; and the same may be said of the other classes 

 of moUusks, and more especially of the Cephalopoda. Of all the 

 various forms of Paleozoic tetrabranchiates there are barely more 

 than a half-dozen surviving types, and of these one, Orthoceras, itself 

 becomes extinct in this period. But, in the place of these ancient 

 types, we have others of the same class which are no less conspicu- 

 ous for their numbers than for the complexity of form which they 

 subsequently attain, and some of which exhibit a marked advance 

 upon their predecessors in the scale of organisation. The ammo- 

 nites, whose advent appears to have been foreshadowed in the 

 goniatite and the Devonian Clymenia, now for the first time 

 acquire any importance, and, indeed, if we except certain forms 

 from the Carboniferous deposits of India and Texas — Arcestes, 

 Xenodiscus, Sageceras, Medlicottia— now for the first time appear 

 altogether. The numerous species which in some districts, more 

 especially in the region of the Alps, crowd the deposits of this age, 

 belong in principal part to the families Arcestidse and Pinacocera- 

 tidae, as representatives of the leiostracous, or smooth-shelled divi- 

 sion, and the Tropitidae. Ceratitid®, and Clydonitidae (with the 

 somewhat aberrant genera Cochlocei-as, Rhabdoceras, Choristoceras, 

 and Clydonites), of the Trachyostraca, or forms with strongly sculp- 

 tured shells. 



In these deposits, also, we meet in the Belemnitidse with the fii-st 

 unequivocal traces of the dibranchiate or two-gilled, cephalopods, 

 which, if we except the Nautilus, alone of this class of mollusks 

 inhabit the seas of the present day.* The rugose and tabulate 



* The view cntsrtaincd by severul eminent paleontologist'', that the .ira- 

 monitcs tliemselvea represent dibranchiate forms, requires further support 

 before it can be fully accepted. The evidence at the present time appoars to 

 be fully as much, if not more, opposed to this notion as it is in favor of it. 



