9ii 



CHAPTER XLVI. 



CLASS PISCES. 



General Structure. 



The members of the class Pisces, commonly known as Fishes, form 

 the first division of Professor Huxley's Ichthyopsida, and are gener- 

 ally characterised by living in water ; breathing by branchiae, or 

 gills, throughout life ; having the heart furnished with a single ven- 

 tricle and auricle (atrium); having the limbs, when present, in the 

 form of fins ; being provided with unpaired median fins supported 

 by fin-rays ; and by the skin being either naked, or covered with der- 

 mal scales or bony scutes. There is no amnion or allantois devel- 

 oped in the embryo, and the reproduction is nearly always oviparous. 

 Certain forms do not, however, exhibit all the above features, and 

 the relation of the more generalised Fishes to the Amphibia is very 

 intimate. The peculiar system of mucous canals and the lateral 

 line are highly characteristic of Fishes, although they are not 

 invariably present. 



Before noticing such features as are of especial importance to the 

 palaeontologist, it will be convenient to mention that according to 

 the arrangement adopted in this work the class is divided into the 

 following six orders — viz., Cyclostomi, Elasmobranchei, Chimeroidei, 

 Dipnoi, Ganoidei, and Teleostei, of which the salient features will 

 be noticed in the succeeding chapter. Dr Giinther has, indeed, 

 proposed to bracket together the second, third, fourth, and fifth 

 orders as a subclass under the name of Palaeichthyes, ranking as 

 equivalent to the Cyclostomi and Teleostei. Professor Huxley, Dr 

 Traquair, and others have, however, shown that the Ganoidei are so 

 intimately connected with the Teleostei, while the Elasmobranchei, 

 Chimeroidei, and Dipnoi differ in so many respects among them- 

 selves and from the former, that such a grouping does not appear 

 consonant with their true relationship. 



Another scheme, proposed by Professor Cope and adopted by 



