46. SALMONID^ SALMO. 311 



Finally, to complete our onuineraJion of these variable characters, we must mention 

 that iu 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 iu the skin. 



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

 found to bo constant in numbers of individuals, and iu which it is difficult to i)erceive 

 Bigus of modilication due to external circumstances. 



Snch characters, according to the views of the zoologists of the present age, are 

 Buiiticieut for the definition of species ; at all events, in every description they ought 

 to be noticed, and the confused and unsatisfactory state of our kuowleilge of Sal- 

 monoids 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 

 liere. 



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



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

 and iu females the maxillary is pro[)ortiouately shorter than iu the .adult. 



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



4. The arrangement aud permanence of the vomeriue teeth. 



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

 arc often lost, aud their absence iu a species usually provided with them is not un- 

 common. 



6. The form of the caudal flu in specimens of a given size, age, or sexual develop- 

 ment. 



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

 above the lateral line. The scales of the lateral line are always more or less enlarged 

 or irregular aud the number of scales should be ascertained higher up ; this is one of 

 the most constant aud valuable of all the specific characters. 



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

 mens from the same locality. 



9. The number of vertebne. 



10. The numlter of pyloric cceca. 



11. The number of gill-rakers. 



The species may be divided as follows : 



•Marine salmon, anadroraous, with the vomerine teeth little developed, those on the 

 shaft of the boue few and deciduous; scales large (Lat. 1. 120) ; caudal well 

 forked, truncate in old individuals; no hyoid teeth; sexual differences 

 strong; breeding males with the lower jaw hooked upwards, the upper 



jaw emarginate or perforate to receive its tip (Salmo) salar. 



•* River salmon, not anadromous, with the vomerine teeth largely developed, those on 

 the shaft of the bone numerous, persistent, in one zigzag row or two alter- 

 nating i"ows; sexual ditfereuces not strongly marked, the male with the 

 7iremaxillaries somewhat enlarged. (Fario Valenciennes.) 

 a. Ilyoid bone entirely toothless. 



b. Scales large, 120-150 in a longitudinal series. 



c. Caudal iiu forked irideus. 



cc. Caudal fin truncate gairdneri. 



hb. Scales small, 165-205 in a longitudinal scries. 



d. Caudal fin truncate npilurus. 



aa. Hyoid bone with a narrow band of small teeth (often lost) ; scales small (lat. 1. 

 155 to lUO) purpuratus, 



•Marine salmon, anadromous, with the vomerine teeth little developed, those on the 

 shaft of the bone few and deciduous; scales largo (Lat. 1. 120) ; caiulal fin well 

 forked, truncate in old individuals; no hyoid teeth ; sexual diflereiici's strong; 

 breediug males with the lower jaw hooked upwiirds, the ux»per jaw emargi- 

 nate or perforate to receive its tip. (Salmo.) 



