Jordan and Rvermann. Pishes of North America. 485 



weight of 14 or 16 pounds. Such large river trout are frequently named 

 or described as Salmon trout, Bull trout, Steelheads, etc. 



The proportion of the various parts of the body to one another vary 

 exceedingly, in the same species, with age, sex, and condition. 



The fins vary to a certain degree. The variation in the number of rays 

 in any one genus (except Oncorhynchus) is inconsiderable, and of no value 

 for specific distinction. Although some species appear to be distinguished 

 by a comparatively low dorsal and anal fin, yet the proportion of the 

 height of these fins to their length is a rather uncertain character. In 

 most of the species the fin rays are longer during the stages of growth 

 or development. The caudal fin especially undergoes changes with age. 

 Young specimens of all species have this fin more or less deeply excised, 

 so that the young of a species which has the caudal emarginate through- 

 out life is distinguished by a deeper incision of the fin from the young 

 of a species which has it truncate in the young state. The individuals 

 of the same species do not all attain to maturity at the same size. 



Finally, to complete our enumeration of these variable characters, we 

 must mention that in old males, during and after the spawning season, 

 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 to external cir- 

 cumstances. 



Such characters, according to the views of the zoologists of the present 

 age, are sufficient for the definition of species ; at all events, in every 

 description they ought to be noticed, and the confused and unsatisfactory 

 state of our knowledge of Salmonoids is chiefly caused by authors having 

 paid attention to the more conspicuous but unreliable characters, and but 

 rarely noted one of those which are enumerated here : 



1. The form of the preoperculum of the adult fish. 



2. The width and strength of the maxillary of the adult fish. In young 

 specimens and in females the maxillary is proportionately shorter than in 

 the adult. 



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 pro- 

 vided 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 trans- 

 verse rows above the lateral line. The scales of the lateral line are always 

 more or less enlarged or irregular and the number of scales should be 

 ascertained higher up ; this is one of the most constant and valuable of 

 all the specific characters. 



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

 of specimens from the same locality. 



