84 FISHES. 



less symmetrical, the last vertebra occupying a central posi- 

 tion in the base of the fin, and being coalesced with a flat 

 osseous lamella, the hypural (Fig. 23, 7o), on the hind margin 

 of which the fin-rays are fixed. The hypural is but a union 

 of modified hsemapophyses which are directed backwards, 

 and the actual termination of the notochord is bent upwards, 

 and lies along the upper edge of the hypural, hidden below 

 the last rudimentary neural elements. In some Teleosteans, 

 as the Salmonidce, the last vertebrae are conspicuously bent 

 upwards : in fact, strictly speaking, this homocercal condition 

 is but one of the various degrees of heterocercy, different from 

 that of many Ganoids in this respect only, that the caudal fin 

 itself has assumed a higher degree of symmetry. 



The neural and hoemal arches generally coalesce with the 

 centrum, but there are many exceptions, inasmuch as some 

 portion of the arches of a species, or all of them, show the 

 original division. 



Tlie vertebrae are generally united with one another by 

 zygapophyses, and frequently similar additional articulations 

 exist at the lower parts of the centra. Parapophyses and 

 ribs are very general, but the latter are inserted on the centra 

 and the base of the processes, and never on their extremities. 

 The point of insertion of the rib, more especially on the 

 anterior vertebrae, may be still higher — viz. at the base of the 

 neural arch, as in Cotylis and allied genera, and even on the 

 top of the neurapophysis, as in Batrachus. 



There is a great amount of variation as regards the degree 

 in which the primordial cranium persists ; it is always more 

 or less replaced by bone ; frequently it disappears entirely, 

 but in some fishes, like the Scdmonidm or Esocidce, the carti- 

 lage persists to the same or even to a greater extent than in 

 the Ganoidei holostei. Added to the bones preformed in car- 

 tilage are a great number of mcmhrane-honcs. The different 

 kinds of these membrane-bones occur with greater or less 



