[Fig. 118].—Teeth of Cretaceous Sharks (Otodus and Ptychodus).—After Leidy. their predecessors ([Fig. 121]). It is interesting to observe that these old-fashioned fishes had culminated before the advent of air-breathing Vertebrates, which appear for the first time in the Carboniferous. It is further to be observed that groups of fishes furnished with means of aiding their gills by rudimentary lungs were especially suited to waters more charged with carbonic acid, and less with free oxygen, than those of more recent times. This remark especially applies to the mephitic and sluggish streams and lagoons of the Carboniferous swamps, where, in the midst of a rank vegetation and reeking masses of decaying organic matter, the half air-breathing fishes and the amphibious reptilian animals met with each other and found equally congenial abodes. Thus, independently of the fact that some of these fishes were probably vegetable feeders, it is not altogether an accident, but a wise adaptation, that caused the culmination of the reptilian fishes and batrachian reptiles to coincide with the enormous development of the lower forms of land-plants in the Devonian and Carboniferous. Another curious illustration of the diminishing necessity for air-breathing to the fishes, is the change of the tail from the unequally-lobed
[Fig. 119].—Tooth of a Tertiary Shark (Carcharodon). or heterocercal form, which prevailed in the Palæozoic, to the more modern equally-lobed (homocercal) style in the Mesozoic. The former is better suited to animals which have to rise rapidly to the surface for air, and is still continued in some modern fishes, which for other reasons need to ascend and descend, or to turn themselves in the water; but the homocercal form is best suited to the ordinary fish, whether Ganoids or Teleosts ([Fig. 122]). It is curious also to find the beginning of the dominancy of the ordinary fish to coincide with that of the broad-leaved exogenous trees in the later Cretaceous, and to precede immediately the appearance of the mammals on the land; all these changes being related to the purer air, the clearer waters, and the more varied continental profiles of the later geological periods. Thus physical improvement and the changes of animal and vegetable life are linked together by correlations which imply not only design, but prescience, whether we attribute these qualities to a spiritual Creator or to mere atoms and forces.
Fig. 120.—A Liassic Ganoid (Dapedius). Restored.—After Nicholson.
Fig. 121.—Cretaceous Fishes of the modern or Teleostian type.
a, Beryx Lewesiensis. English chalk. b, Portheus molossus (Cope). A large fish from the American Cretaceous. One twenty-eighth natural size.