ICHTHYOLOGY. 



209 



Introduc- prey, cartilaginous arches and spines are added above tlie 

 tion. " chorda," in the membranous wall of the neural canal, 

 Osteology, and in the tail also beneath it. In the Sturgeon and Chi- 

 ^— ^/'■~-' nisra the bases of" the cartilaginous arches inclose the 

 " chorda." In the Lepidosiren, the neural and haemal arches 

 and their spines are ossified, but the centrums are still con- 

 fluent as a dorsal membrano-gelatinous chord ; and Agassiz 

 lias shown that this was the condition of the vertebra." of 

 the most ancient fossil fishes — those of the Silurian and 

 Devonian rocks. In many Sharks and Uays the "chorda" 

 is encroached upon by osseous or cartilaginous convergent 

 laminse, and by concentric, successively shorter, centri- 

 petally developed cylinders, and is thus reduced to a string 

 of gelatinous beads, each l)ead occupying the interspace 

 between the opposed concave surfaces of the vertebral 

 bodies. This nioniliform state of the " chorda" is per- 

 sistent in most osseous fishes ; whilst in some others the 

 gelatinous biconical segments of the "diorda" are insulated 

 by the comjileted centripetal process of ossification ; and 

 in one genus, tlie Lepidosteus, they are converted into 

 osseous balls, fixed to the fore part of each vertebral body, 

 which plays in the concavity or cu]) of the \ertebra next 

 in advance. The slightly compressible jelly-bags that fill 

 the concave opposing articular surfaces of the vertebra; are 

 admirable contrivances for facilitating the motions of the 

 fish. The neural and haemal arches and spines are bony in 

 all osseous fishes ; and in all fishes cliondrification and ossi- 

 fication of the vertebral colimm conmiences in these arches. 

 The skeleton of fishes presents many instances of what 

 Owen terms " vegetative repetition of perivertebral parts." 

 There being no distinct neck in fishes, we find no peculiar 

 cervical vertebras, the series posterior to the head consist- 

 ing of abdominals and caudals. With the former the ribs 

 are connected, and in the latter the haemal canal is formed 

 by the deflection and union of the parapophyses, as men- 

 tioned above. The pleurajiophyses, or ribs, are generally 

 articulated in fishes to the ends of the parapophyses, but in 

 Platax and Poljiplerus they articulate with the bodies of 

 the vertebrae. In the Salmon and Dory they continue to be 

 attached to some of the parapo[)hyses after these are bent 

 down in the tail to form the ha;mal canal and haemal spines. 

 The DiodoiiltdfB, Ostratioiiidic, Pislulartdce, Si/iifjiiatliido:, 

 Cijclopferid(P, and Lopliiidce, are destitute of ribs. 



Slender sujiplementary ribs often issue from near the 

 head of the ribs, passing outwards and backwards from be- 

 tween the lateral muscles. Professor Owen names them 

 " epipleural spines." They are highly developed in the 

 Sa/jiionidre, Cliipeid/e, and Scombei-iiUe. In the common 

 Herring, dermal bones are connected with tlie lower ends 

 of the ribs, and have been called, though erroneovisly, ster- 

 nal bones, since they do not belong to the endo-skeleton. 



In osseous fishes, imposed above the proper nein-al spines, 

 there is a second row of spines belonging to the dermo- 

 skeleton, of greater or less extent, and serving to support 

 the dorsal fin or fins. From their bases other spines of an 

 inverted shape usually descend into the intervals of the 

 neural spine-;, and are hence named by Owen " interneural 

 spines." These interneurals are sometimes in lateral pairs, 

 as in the PleiiroiiectidfE, the Chmlodontidce, Zeidce, and 

 some other compressed fish. 



The modifications of the dorso-interneural and dermal 

 spines are repeated on the ventral aspect, posterior to the 

 anus, producing inter-ha?nial and dermo-haemal spines which 

 support the anal fin. Each dermal spine or ray is either 

 readily divisible into two laterally, or presents a furrow iiv 

 dicative of a tendency to such a division. The caudal fin 

 is extended also on compressed intercalary and dermal 

 spines, diverging from a 'imvi coalescent and abbreviated 

 caudal vertebrae. In the base of the caudal formed of these 

 elements, the haemal spines are more developed than the 

 neural ones in very many species, as in the Salmonidce, and 



VOL. XII. 



then they press upwards the short aud almost obsolete verte- Introduc- 

 bral centra which form the tip of the spinal colunni, and into ''""■ 

 which the spinal marrow is not extended. This larijer de- Osteologj-. 

 velopment of the ha;mal processes elongates the upper lobe '^"^-'"^ 

 of the fin, and gives a very unsymmetrical form to the 

 caudal of Sharks, Stm-geons, and Elephant fish. Fishes pos- 



Fig. 11. 

 Hemisc-tfilium trispecularey Australian sliaik of tlio family of ScyltUdcr. 



sessing such a caudal are named by M. Agassiz " hetero- 

 cercal ;" and those with a symmetrical caudal fin he deno- 

 minates " homocercal," inchiding, however, in the latter 

 group the existing fish wherein tlie obliqueness of the cau- 

 dal vertebrae does not affect, in a decided way, the shape 

 of the caudal fin, the SulinoiiidcB, for instance, being homo- 

 cereals. 



Among the palaeozoic fishes the heterocercal fishes greatly 

 predominate ; all that have been found in the magnesian 

 limestone, and in older strata, having the superior lobe of 

 the tail prolonged. The only existing osseous fishes that 

 are recognised by M. Agassiz as heterocercal belong to the 

 genus Lepidosteus. 



The size, number, and shape of the vertical fins depends 

 on the de\'eloj)ment and grouping of the accessary and in- 

 tercalary spines, the proper neural and haemal spines of the 

 endo-skeleton showing no direct indication of the existence 

 or position of those fins; but it is convenient to notice them 

 in connection with the vertebral column, as the systematic 

 arrangements of ichthyologists have had much reference to 

 the mmiber, relative position, and condition of the rays of 

 those fins, in regard of their being simple and sjiinous, or 

 jointed and forked. We have already noticed the terms 

 Malacopterygian and Acanthopterygian as deri\ed from 

 these varieties of structure. Species occur among the 

 Gobiidre and some other families of fish, in which there is 

 much difficulty in deciding to \vhich of the two groups they 

 belong. In the Laneelet and Lamprey, the dorsal and cau- 

 dal fins are simply cutaneous folds, with scarcely distin- 

 guishable soft fibres for rays. Synhranchiis gutlurnlis, and 

 (Jliannomurana, have nearly obsolete rays at the tip of the 

 tail only; and in Gi/muomurana the folds as well as rays 

 are wanting. The interneural and dermal spines seldom 

 coincide in number with the neural spines. They are often 

 more numerous, but more frequently less numerous. The 

 sucking apparatus on the head of the Remora is an assem- 

 blfige of peculiarly modified and connected dermal spines. 

 Great development of the dermal spines often renders 



Fie. 12. 



Ccntrlscits httrin'rosus of tlie Fi.tltilar/'hc, roprcscntinp in tlio Australian seafe 



illy Mediterranean "Snipo-iisli." 



them powerful weapons of defence, as in Tracliinus. The 

 Chiinecrce, Cestracions, Piked Dog-fish, and Balisles, are 



2 D 



