Ixx Introduction. 



which are nearly allied to them, by the stunting- of the neural in 

 correspondence with the greater development of the haemal arches. 

 This form of tail may to a superficial examination appear quite 

 equilobed, and it is ordinarily spoken of as ' homocercal.' It is, 

 however, morphologically ' heterocercal,' as the haemal and neural 

 arches enter into its composition in very unequal proportions, the 

 chorda dorsalis being really prolonged to the upper angle of the 

 tail fin, and the ' hypural ' plates being all modified haemal arches, 

 and not half of them haemal, and half of them neural ossifications. 

 A true ' diphycercal ' tail is finally produced in the Acanthopteri, 

 by a reduction of the disproportionate size of the haemal, and by 

 a simultaneous stunting of the central elements of the terminal 

 vertebrae. 



The diff'erences in the structural arrangements of the skulls of 

 Fishes are very much greater than those observable in the skulls of 

 members of any other Vertebrate class, relating as they do to points 

 of no less morphological and indeed j)hysiological importance than 

 the absence or presence of cranial bones; of freely movable gill- 

 covers, and suspensoria; of maxillary and premaxillary, and of 

 mandibular bones. In the Elasmohrmicldi and Mars'ipohranchii^ 

 there are no cranial bones ; and with regard to the praemaxillary 

 and maxillary bones, it can only be said, that the sites which those 

 membrane-bones occupy in other fishes may, perhaps, be considered 

 as marked out in these orders by the presence of certain labial 

 cartilages. The Cidmaerae diflfer from the other Elasmohranchii, 

 the Sharks and • Rays, in having a movable operculum and an 

 immovable suspensorium, and in this latter particular the non- 

 mandibulate Marsipohranchii more or less closely resemble them. 

 The Teleostei and Ganoidei differ from these Fishes by possessing 

 cranial, maxillary, praemaxillary and opercular bones. Their oper- 

 cula and suspensoria are always movable. 



In the Masnwhranchil the skull is distinctly articulated to the 

 first trunk vertebra. In the Chondrosteal Ganoidei, the largely 

 developed parasphenoid reaches for a considerable distance back- 

 wards underneath the anterior vertebrae ; whilst in the Holostean 

 Ganoidei and many Physostomous Teleostei, the first and some of 

 the following vertebrae may be suturaily connected with the basi- 

 occipital. In other osseous Fish the basi-occipital presents a concave 

 conical cavity for apposition with the similar one upon the anterior 



