MOLLUSCA 



451 



in the rocks of this system ; as examples, may be mentioned Pro- 

 ductus, Athyris, and Cyrtina. Koninckina is a new genus of the 

 Spirifer family, which is confined to the Trias. The still existing 

 genera, Terebratula and Rhynchonella, are much the most abun- 



Plate VIII. American Trtassic Fossils 



1. Monotis subcircularis. 2. Myophoria alta. 3. Trachyceras Whitneyi, 1/2. 

 4. Arcestes Gabbi. 5. Clathropteris platyphylla. 6. Otozamites latior. (Figs. 1-4 

 after Gabb. Figs. 5, 6 after Newberry.) 



dant brachiopods of the period, and Thecidium, which later be- 

 comes important, has its beginning here. 



Mollusca. — Almost in proportion to the decline of the brachio- 

 pods is the rise of the Pelycypoda, or Bivalves, which now become 

 far more varied and abundant than they had been in the Palaeozoic. 

 Pecten, Monotis (VIII, 1) , Myophoria (VIII, 2) , Halobia, and Car- 



