592 ON RACES AND HYBRIDS AMONG THE SALMONID^. [DcC. 2, 



On November 14th I examined a male Lochleven ket, 20 inches 

 in length, its head being 4'2 inches long, tail-fin TS inch and rounded 

 at its extremity, depth of body 4*2 inches ; a considerable-sized 

 knob on the end of the mandibles, which, when the mouth is closed, 

 is outside the upper jaw but against which it rubs, and where it has 

 formed a sore. The posterior edge of the orbit is exactly in the 

 centre of the length of the head, while the length of the prenasal 

 portion of the head equals H the diameter of the eye, or is nearly 

 similar to what obtains in the female Otago Trout. This instance 

 is given as a typical form of the Lochleven Trout, wliile the 

 heads of the Brook-Trout correspond with those of the Lochleven 

 variety. 



It appears to me probable that the sexual differentiation in the Trout 

 is similar to what occurs in the Salmon, wherein in large examples the 

 prenasal portion of the head equals the distance between the hind 

 edge of the preopercle and the hind edge of the orbit. But as these 

 proportions differ from what is seen in smaller, or rather perhaps 

 one should say younger examples, the following may be pretty 

 readily traced out in specimens. 



After the second year, or when the fish commences breeding, a 

 knob appears at the end of the lower jaw, which knob yearly 

 increases in size at the breeding-season, until in /-years-old fish (as at 

 Howietoun) it is in advance of the upper jaw when the mouth is 

 closed, often forming a sore surface in front of the premaxillaries. 

 At such times the mandibles have increased in length proportionately 

 more than the bones of the snout ; thus the prenasal portion will only be 

 found to be one half of the comparative length to what obtains in old 

 fish, as seen in the Otago example. Here at Howietoun my ob- 

 servations had to be stopped, as fish over 7 or 8 years of age are 

 not kept, being somewhat sterile. But it is clear that in the very 

 large male specimen of which the head is figured the bones of the 

 upper jaw have grown so that, instead of the knob on the mandibles 

 being in advance of the upper jaw, it is inside the mouth. 



In examining the Otago fish I found the mandibular hook half 

 an inch long, extending not only on the upper surface, but also on 

 the front and slightly on the inferior surface of the symphysis, while 

 superiorly it becomes received into a large depression, situated 

 between the ascending portions of the two premaxillaries, from the 

 sheath of which it is divided by the mucous membrane of the mouth 

 and soft lining of the palate. The two ascending portions of the 

 premaxillaries are not fixed one to the other, but can move freely 

 and be more or less easily separated ; while the membrane which 

 binds them down, although strong, is loose, permitting of con- 

 siderable motion. Consequently the hook, although it may press 

 against the palate, can do so without occasioning any injury, for the 

 parts give way before it. But of course if any irritation occurs at this 

 spot and ulceration supervenes, the hook' might pass through and 

 appear on the upper surface of the head. When this takes place, 



^ Whether this hook ever falls off, or ulcerates off, or is partially or wholly 

 absorbed after each breeding-season, I have no personal knowledge. 



