THE JURASSIC PERIOD. 91 



the Triassic fauna which occupied the same region previously. It has the aspect 

 of the European Liassic fauna, and of a similar fauna found in the island of Timor, 

 between Java and Australia, and also in Argentina. 



As the successive horizons of the European Jurassic are defined most char- 

 acteristically by their ammonites, 1 the most instructive element of the fauna of 

 this stage in the Nevada-California-Oregon province was the ammonite family 

 Arietidce, represented by Arnioceras nevadanum, A. humboldti, A. woodhulli, 

 Coroniceras claytoni, and Vermiceras crossmani. The belemnites are represented 

 by a single form. Several genera of pelecypods were present (Goniomya, Lima, 

 Pecten, Pinna, Plicatella, Pleuromya, and Pholadomya); a Turbo represented 

 the gastropods, a Cidaris the echinoderms, and a Glyphcea the crustaceans. 

 This list appears very meager when compared with the nearly 250 genera and 

 more than 1600 species enumerated by Etheridge from the corresponding 

 European fauna. How this fauna had communication with central Europe, 

 Timor, and South America is undetermined. A route via the " central Mediter- 

 ranean Sea" of Neumayr has been suggested, 2 and a route from Timor, via New 

 Zealand and Antarctica to South America, and thence by the coast to California, 

 may be speculatively offered as involving not improbable geographic connec- 

 tions. 



The American fauna of the Middle Jurassic epoch is not sufficiently ample, 

 as now known, to clearly indicate its relations to foreign faunas, but it has the 

 aspect of the central European fauna (J. P. Smith). Like the preceding, it 

 is essentially a group of molluscan forms in which the pelecypods greatly out- 

 number all other species. Several of the preceding genera were present, and 

 several new ones were added (Modiola, Mytilus, Pinna, Pteroperna, Gervillia, 

 Lima, Ctenostreon, Pecten, Pholadomya, Trigonia, Opis, Inoceramus). The 

 cephalopods embraced ammonites (Sphceroceras , Grammoceras, and Perisphinctes) 

 and a belemnite. The gastropods were represented by a large NeHnea and the 

 brachiopods by Terebratula and Rhynchonella. 



In the fauna of the Upper Jurassic, the molluscan monotony is relieved by 

 the introduction of several species of corals which are so similar to European 

 species of the Corallian formation as to imply equivalence with that horizon. 

 This is confirmed by the species of pelecypods, by the cephalopod Rhacophyllites, 

 and by the gastroped Chemnitzia. In other beds of the series, a more consider- 

 able group of pelecypods (Aucella, Avicula, Amusium, Trigonia, Entolium, Oxy- 



1 " These highly specialized faunas, as has been pointed out by several of the most 

 distinguished paleontologists in Europe, must have been extremely sensitive to the 

 influences of the changes of their surroundings in passing from one geological horizon 

 to another, and have recorded these mutations in their own organizations. Even 

 the encyclopedic Quenstedt continually expresses his satisfaction in turning from 

 the uncertain indications afforded by the more generalized structures of other mollusca 

 to the decisive chronologic evidence usually given by the fossils of this group." Hvatt, 

 Geology of the Taylorville Region, Bull. Geol. Soc. Am., Vol. Ill, p. 404. 



2 For a discussion of this and related subjects see " Mesozoic Changes in Fauna! 

 Geography," by James Perrin Smith, Jour Geol., Vol. Ill, 1895, pp. 369-384. 



