Page:Popular Science Monthly Volume 27.djvu/633

Rh designation, for it is highly probable that it was from one of the early orders of fishes that the Batrachia took their rise. Omitting from consideration the lowest vertebrata, the sand-lances and lampreys, which are not fishes, we have remaining a body of animals which present



great varieties of structure. Of the four great sub-classes into which they naturally fall, but one can be called true fishes. The others embrace the sharks, the chimæras, and the lepidosirens. It is interesting to note that these four divisions are more closely approximated during the Permian period than at any later time. An order technically referred to the sharks, and known as the Ichthyotomi, combines many of the characters found separately in three of the sub-classes. The creatures which especially deserve the name of batrachian fishes, the ceratodonts, etc., also abounded during the Permian period. From this time the true fishes began to run their course. They have peopled all waters, and have branched into a greater diversity of form