46. SALMONID^ SALMO. 
311 
Finally, to complete onr 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 modilicatiou due to external circumstances. 
Such characters, according to the views of the zoologists of the present age, are 
sufficient for the delinition of species ; at all events, in every description they ought 
to he 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 
hut 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 arrangemeut and permanence of the vomerine teeth. 
6. The development or absence of teeth on the hyoid bone. In old examples these 
are often lost, and their absence in a species usu.ally provided with them is not un- 
common. 
6. The form of the caudal fin in simcimens 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 iri’egular and the number of scales shouhl be ascertained higher up; this is one of 
the most coust.alit 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 vertebrae. 
10. The number of pyloric cceca. 
11. The number of gill-rakers. 
The species may be divided as follows : 
* 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 well 
forked, truncate in old individuals; no hyoid teeth; sexual differences 
strong ; breeding males Avith the lower jaw hooked upwards, the upper 
jaw emarginate or perforate to receive its tip (Salmo) salar. 
** River salmon, not auadromous, with the vomerine 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. 
bb. Scales small, 165-205 in a longitudinal series. 
d. Caudal fin truncate spilunis. 
aa. Hyoid bone with a narrow baud 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) ; caud.al fin well 
forked, truncate in old individuals ; no hyoid teeth ; sexual differences strong ; 
breeding males with the lower jaw hooked upwards, the upper jaw emai'gi- 
nate or perforate to receive its tip. (Salmo.) 
