192 ZOOLOGY SECT. 



are distinguished from Elasmobranchs and Holoeephali by having 

 the primary skull and shoulder-girdle complicated by the addition 

 of investing 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 in most cases no cerebral hemispheres, but an undivided 

 prosencephalon. 



1. EXAMPLE OF THE SUB-CLASS. THE BEOWN TROUT 



(Salmofario). 



The Brown Trout is common in the rivers and streams of Europe, 

 and has been acclimatised 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 



n 



FIG. 865. Salmo fario. a. 1. adipose lobe of pelvic fin ; an. anus ; c. /. caudal fin ; d. f. 1, 

 first dorsal fin ; d.f. 2, second dorsal or adipose fin ; 1. 1. lateral line ; op. operculum ; pet. f. 

 pectoral fin : pv. f. pelvic fin ; r. /. ventral fin. (After Jardine.) 



grow to nearly a metre in length. Other, species of Salmo, such 

 as the Salmon (S. solar], the Lake Trout (S. ferox), the American 

 Brook Trout (S. fontinalis), are common in the Northern Hemi- 

 sphere and differ only in details from S. fario. 



External Characters. The body (Fig. 865) is elongated, 

 compressed, 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. 866, 

 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 vomerine teeth. The lower jaw (md.) is mainly supported by 

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

 each ramus of the mandible is bounded mesially by a deep groove. 

 The floor of the mouth is produced into a prominent tongue (t.) 

 bearing a double row of teeth. In old males the apex of the lower 

 jaw becomes curved upwards like a hook. 



