348 ZOOLOGY 



Photograph from Am. Mus. Natural History 

 FIG. 131. Shovel-nosed sturgeon. 



The bony 4. Coming now to the typical bony fishes, or Teleos- 



tei, we find a bewildering array of families, genera, and 

 species, both in fresh water and in the sea. Although 

 certain fishes, such as the salmon, live in both fresh and 

 salt water, the marine fishes are in general quite differ- 

 ent from those of rivers and lakes. The great develop- 

 ment of the modern families seems to have taken place 

 at the end of the Mesozoic time, when the sea invaded 

 large parts of the northern continents. In those days 

 the whole Mississippi Valley, to the very bases of the 

 present Rocky Mountains, was a great shallow sea, warm 

 and eminently fitted for the growth and development of 

 diverse animals. Some of the fishes were very large, the 

 giant Hypsodon (or Portheus) exceeding any modern spe- 

 cies of similar type. The scales show us that the fauna 

 was not so diversified as the modern one, and it was not 

 until the Tertiary that a number of the higher forms 

 came into existence. No doubt the various families 

 originated in different areas, and it was not until much 

 later that many of them spread over the waters of the 



