Zöology; a textbook for colleges and universities . It is to this type that the term ganoid has been more especially restricted in recent years. The bowfin (Amia caka), also of the MississippiValley, is actually nearer to the garpike than the latteris to Polypterus, though its scales are not ganoid. It is,however, a and isolated type, and althoughthe scales superficially resemble those of many of thehigher fishes, the fine fibrilloe or threads composing thebasal part run lengthwise as they do in the lung-fishes. The sturgeons (Chondrostei) constitute another iso- sturgeonslated t
Zöology; a textbook for colleges and universities . It is to this type that the term ganoid has been more especially restricted in recent years. The bowfin (Amia caka), also of the MississippiValley, is actually nearer to the garpike than the latteris to Polypterus, though its scales are not ganoid. It is,however, a and isolated type, and althoughthe scales superficially resemble those of many of thehigher fishes, the fine fibrilloe or threads composing thebasal part run lengthwise as they do in the lung-fishes. The sturgeons (Chondrostei) constitute another iso- sturgeonslated type surviving from ancient times. They havelarge, bony plates on the body, and the tail is heterocer-cal- - that is to say, bends upward at the end, carryingthe fin on the lower side. This is a feature also ob-served in the sharks, and less conspicuously in thebowfin and garpike. It will be noted that these groupsof archaic fishes exist in fresh water, in the large riversystems of continental areas, but not in the sea. 348 ZOOLOGY The bonyfishes. Photograph from Am. Mus. Natural HistoryFIG. 131. Shovel-nosed sturgeon. 4. Coming now to the typical bony fishes, or Teleos-tei, we find a bewildering array of families, genera, andspecies, both in fresh water and in the sea. Althoughcertain fishes, such as the salmon, live in both fresh andsalt 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 placeat the end of the Mesozoic time, when the sea invadedlarge parts of the northern continents. In those daysthe whole Mississippi Valley, to the very bases of thepresent Rocky Mountains, was a great shallow sea, warmand eminently fitted for the growth and development ofdiverse animals. Some of the fishes were very large, thegiant Hypsodon (or Portheus) exceeding any modern spe-cies of similar type. The scales show us that the faunawas not so diversified as the modern one, and it was notu
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