But when, as in the majority of species, the hinder border is emarginate, so as to form an upper and a lower lobe, the former is never known to contain any extension of the spine; although some South American Goniodonts have the upper ray prolonged into a sort of filament, yet in other forms the lower ray is similarly elongated, and neither can be compared with the true filament of the young gar or the upper lobe of sturgeons and sharks.
It may not be possible to draw a sharp line between the tail of most adult Teleosts, and that of Amia and Lepidosteus, but perhaps the old term homocercal can be employed for the former.
Upon the whole, it would appear that the tail of the youngest Lepidosteus is protocercal like those of the lowest vertebrates and the generalized forms called Dipnoans; that the second or obviously heterocercal stage is comparable with the tails of sharks and sturgeons, while the last stage seems to correspond quite closely with that of the teleostean embryo. And, as the Teleosts are almost universally regarded as the most specialized group of fishes, there appears to be a pretty close agreement between the successive stages of Lepidosteus and the rank of the forms or groups with which comparison has here been made.
The corresponding geological series is less complete and satisfactory. No forms resembling Amphioxus or the hag-fishes and lampreys have yet been found fossil, although all, excepting the former, have horny teeth, of which, it would seem, some traces might well be preserved.
But among the oldest fishes are some described by Huxley whose tails are apparently protocercal. The resemblance to the earliest stage of Lepidosteus is emphasized also by the existence of two dorsals and two anals.
Fossil species of Amia and Lepidosteus have recently been discovered by Prof. Marsh in the Tertiaries of Western America. The Megalurus of the European rocks had a tail strongly resembling that of Amia, but this kind of tail is not known among the palæozoic rocks, and Teleosts are first found in the Cretaceous, becoming more and more numerous up to the present time.
But among the earliest known fossil fishes are some in which the end of the spine is not at all bent up; the tail is protocercal. And, with two genera (Glyptolæmus and Gyroptychius) described by Prof. Huxley, it may be possible to determine the correspondence between the two dorsals and anals and the two pair of differentiated spots upon the primordial median fin of the youngest Lepidosteus.
So far as the writer is able to ascertain, the protocercal tail is less frequent in later geological epochs, while the obviously heterocercal form, as with Palæoniscus, etc., becomes more and more abundant.
Apparently, therefore, the order of succession of the three or four kinds of tails coincides, in the main, with the series seen in the grow-
