46. SALMONID.E SALMO. 311 



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 circumstances. 



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 Sal- 

 monoids is chielly 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 preoperculuni 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 preiuaxillaries excepted. 



4. The arrangement and permanence of the voinerine teeth. 



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

 are often lost, and their absence in a species usually provided with them is not un- 

 common. 



6. The form of the caudal fin 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 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 speci- 

 mens from the same locality. 



9. The number of vertebra}. 



10. The number of pyloric cceca. 



11. The number of gill-rakers. 



The species may be divided as follows : 



* Marine salmon, auadromous, with the vonierine teeth little developed, those on the 

 shaft of the bone 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. 



** Eiver salmon, not anadromous, with the vonierine teeth largely developed, those on 

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

 nating rows; sexual differences not strongly marked, the male with the 

 premaxillaries somewhat enlarged. (FARIO Valenciennes.) 

 a. Hyoid bone entirely toothless. 



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



c. Caudal fin forked irideus. 



cc. Caudal fin truncate -. gairdneri. 



l)b. Scales small, 165-205 in a longitudinal series. 



d. Caudal fin truncate sjrilurus. 



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

 155 to 190) purpuratus. 



"Marine salmon, auadromous, with the vomerine teeth little developed, those on the 

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

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

 breeding males with the lower jaw hooked upwards, the upper jaw emargi- 

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



