SECT, xiii PHYLUM CHORDATA 183 



piece of kelp. Nothing is known of the early development : the 

 advanced embryo has elongated gill-filaments (br. /.) projecting 

 through the branchial aperture, a diphycercal tail, and a curiously 

 lobed and nearly sessile yolk-sac (yk. s.). 



Fossil remains of Holocephali are known from the lower Jurassic 

 rocks upwards. As might be expected, they consist mostly of teeth 

 ,and of dorsal fin-spines, but in some cases, and notably in 

 Squcdoraja, practically the whole of the skeleton is preserved. 



Sub-Class III. Teleostomi. 



In this sub-class are included all the commonest and most 

 familiar Fishes, such as the Perch, Pike, Mackerel, Cod, Sole, 

 Herring, Eel, Salmon, etc., as well as the so-called " Ganoid " Fishes, 

 .such as the Sturgeon, Bony Pike (Lepidosteus) and Bow-fin (Amia) 

 of North America, and the Polypterus of the Nile. They are 

 distinguished from Elasmobranchs and Holocephali by having the 

 primary skull and shoulder-girdle complicated by the addition of 

 membrane bones, and by possessing bony instead of horn-like fin- 

 rays. The gills are covered by an operculum ; the anus is distinct 

 from the urinary and genital apertures ; and the brain has no 

 cerebral hemispheres but an undivided prosencephalon. 



1. EXAMPLE OF THE SUB-CLASS THE BROWN TROUT 



(Salmo fario). 



The Brown Trout is common in the rivers and streams of 

 Europe, and has been acclimatized in other parts of the world, 

 notably in New Zealand. It varies greatly in size, according to 

 the abundance of food and the extent of the water in which it lives : 

 it may attain sexual maturity, and therefore be looked upon as 

 adult, at a length of 18 20 cm. (seven or eight inches), but in 

 large lakes it may grow to nearly a metre in length. Other 

 species of Salmo, such as the Salmon (S. solar), the Lake Trout 

 (X ferox), the American Brook Trout (S. fontinalis), are common 

 in the Northern Hemisphere and differ only in details from 

 S. fario. 



External Characters. The body (Fig. 806) is elongated, com- 

 pressed, thickest in the middle, and tapering both to the head and 

 tail. The mouth is terminal and very large : the upper jaw is 

 supported by two freely movable bones, the premaxilla (Fig. 807, 

 pmx.) in front and the maxilla (mx.) behind, both bearing sharp 

 curved teeth arranged in a single row. When the mouth is opened 

 -a row of palatine teeth is seen internal and parallel to those of the 

 maxilla, and in the middle line of the roof of the mouth is a double 

 row of vomerme teeth. The lower jaw (md.) is mainly supported by 

 a bone called the dentary and bears a row of teeth ; on the throat 



