990 REPORT OF COMMISSIONER OF FISH AND FI^4HERIE8. [10] 



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chordal axis, as in the dipbj'cercal Chimwra monstrosn^ or, as in het- 

 erocercal Aniii(rus (Fig. 8), it may, at an early stage, have the chorda 



exserted beyond the last liypural cartil- 

 ages, and at some distance behind them 

 have another hypaxial cartilage developed, 

 which may be called opisthural, as it prob- 

 ably represents the remnant of proximal 

 hypnral pieces, which were developed in 

 some more archaic ancestral form, in which 

 diphycercy was more pronounced or even 

 perfect. Where the caudal ray-bearing 

 fin-fold is developed nearer the end of the 

 chordal axis {Apeltes, Siphostoma, Gamhu- 

 sia) heterocercy is not so pronounced, as 

 the urostyle is shorter, and only a part of the terminal vertelnai are in- 

 volved, whereas in other cases {Salmo, Lepidostcus) more terminal ver- 

 tebrje may be implicated by degeneration. In archaic forms of hetero- 

 cercy there may be some epaxial rays and intermediary supports devel- 

 oped, while the hyi)axial su])ports and rays extend to the end of the 

 upwardly bent termination of the axial column. This trait may possibly 

 differentiate the archaic type of heterocercy {Pala'oniscns, Flatysomv.s, 

 Acipenscr) from the more recent or specialized form {Amiurus) now 

 prevalent amongst Teleosts, and which have for the most part a more 

 or less well-developed urostyle, but with a very short or included opis- 

 thure(= dorsal lobe, Agassiz), and with epaxial spines of the urostyle 

 displaced, rudimentary, or aborted. Outwardly homocercal Paheozoic 

 fishes [Dapedius, Pycnodus) probably had an opisthural filament devel- 

 oped in their larval stages, which subsequently aborted, as in Lejndosteus, 

 but in others {Platysomus, Pygopterus) the terminal part of the chordal 

 axis doubtless became segmented, the segments bearing hypaxial caudal 

 rays and few or no epaxial rays, so that the opisthure was probably rudi- 

 mentary. 



It thus becomes evident that the development of modern Teleosts 

 l)resents only a partial or inexact parallelism with that of the Paleozoic 

 Rhomhoganoidei, for tew, if any, of these forms show the urostyle so 

 distinctly developed or the hypural pieces so extensively co-ossified a^ 

 in existing Teleostei. The Bhomboganoidei, Crossopterygta, Cycloganoideij 

 and Chondrostei show a more decidedtendency towards the development 

 of a continuous dorsal and ventral or only ventral series of caudal rays, 

 and thus trend more towards a diphycercal condition than the existing 

 Teleostei, which may be said to be verging towards hypocercy, when all 

 of the caudal rays will be of hy])axial origin, with frequently a rayless 

 hiatus behind the last hypaxial pieces and the end of an exserted uro- 

 style. These are some of the marks of progress which distinguish the 

 Teleosts and supplement the significant fact of their well-ossified skel- 



