194 THE POPULAR SCIENCE MONTHLY. 



But when, as in the majority of species, the hinder border is emar- 

 ginate, 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 fil- 

 ament, 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 

 Lepiclosteus is protocercal like those of the lowest vertebrates and the 

 generalized forms called Dipnoans ; that the second or obviously hete- 

 rocercal 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 re- 

 garded as the most specialized group of fishes, there appears to be a 

 pretty close agreement between the successive stages of Lepiclosteus 

 and the rank of the forms or groups with which comparison has here 

 been made. 



The corresponding geological series is less complete and satisfac- 

 tory. No forms resembling Amphioxus or the hag-fishes and lam- 

 preys 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 Liepidosteus is emphasized also by the existence of two dor- 

 sals and two anals. 



Fossil species of Amia and JLepidosteus have recently been dis- 

 covered by Prof. Marsh in the Tertiaries of Western America. The 

 Megaluriis of the European rocks had a tail strongly resembling that 

 of Amia, but this kind of tail is not known among the palaeozoic 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 [Glyp)tolmmus and Gyroptychius) described by Prof. 

 Huxley, it may be possible to determine tlie 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 Palceoniscus, 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- 



