The Salmon and Trout 



the skin on the back becomes thickened and spongy, so that the 

 scales are quite invisible or hidden in the skin. 



After this cursory review of variable characters, we pass on to 

 those which we have found to be constant in numbers of individuals, 

 and in which it is difficult to perceive signs of modification due 

 f o external circumstances. 



Such characters, according to the views of the zoologists of 

 the present day, are sufficient for the definition of species; at all 

 events, in every description they ought to be noticed. The confused 

 and unsatisfactory state of our knowledge of the Salmonidce is 

 chiefly caused by authors having paid attention to the more con- 

 spicuous but unreliable characters and who have but rarely noted 

 any of those enumerated here : 



1. The form of the preopercle in the adult fish. 



2. The width and strength of the maxillary in the adult; in 



the young and in females the maxillary is proportionately 

 shorter than in the adult male. 



3. The size of the teeth, those of the premaxillaries excepted. 



4. The arrangement and permanence of the vomerine teeth. 



5. The development or absence of teeth on the hyoid bone. In 



old examples these are often lost, and their absence in a species 

 usually provided with them is not uncommon. 



6. The form of the caudal fin in specimens of a given size, age or 



sexual development. 



7. The size of the scales as indicated by counting the number of 



transverse rows above the lateral line. The scales in the lateral 

 line are always more or less enlarged or irregular, and the 

 number of scales should be counted higher up; this is one 

 of the most constant and valuable specific characters. 



8. A great development of the pectoral fins, when constant in 



a number of specimens from the same locality. 



9. The number of vertebrae. 



10. The number of pyloric caeca, 

 n. The number of gillrakers. 



a. Vomerine teeth little developed, those on the shaft of the bone 



few and deciduous; sexual differences strong; breeding males 

 with the lower jaw hooked upward, the upper jaw emarginate 



or perforate to receive its tip, size large; salar, 163 



aa. Vomerine teeth well developed, those on the shaft of the bone 

 numerous and peristent; sexual difference less marked, but 

 similar in general character to those in Salmo salar. 



b. Scales always small, 150 to 200 cross-series; a large deep red or 



scarlet dash on each side concealed below the inner edge of the 

 dentary bone, this rarely obsolete; mouth large, the maxillary 



162 



