FRESH-WATER TROUT— JAWS AND FINS OP. 187 



salmon (p. 67 ante), this growth, consisting of fibrons connective tissue, increases 

 at the breeding period, and in seven-year-old fish 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. In measuring the proportions of the parts of the head of 

 several specimens I found that whereas the lower jaw was as long or very nearly 

 as long as seen in old fish, the distance from the nostrils to the end of the snout 

 only equalled 1| diameters of the eye, whereas in old salmon it equals 2 diameters 

 of the orbit. At Howietoun my observations had to stop at this point as fish over 

 this age are not kept, but it was clear that should the upper jaw extend in the same 

 proportionate length as in the salmon, the knob would be inside, not outside the 

 mouth. In August, 1883, I received from Mr. Arthur,* of Otago, an old male 

 trout, 8. fario, weighing 21 lb. and 32 in. in length, and this completed my series, 

 the upper jaw had elongated as in the salmon and the hook was now inside the 

 mouth. Whether this soft hook ever completely absorbs is questionable, but the 

 older the fish the larger the knob, and in very old specimens, due to an elongation 

 of the bones of the snout and upper jaw, it is found inside the mouth ; here it may 

 cause ulceration, impossibility of moving the premaxillaries, and death from starva- 

 tion. In the mandible at its anterior end the rami are ossified together without any 

 appearance of a suture. A broad groove (0'3 of an inch) passes from above 

 down the front of the symphysis and having a crest or ridge along each side, this 

 ridge being elevated ahove the plane of the dentary bone. Also anteriorly the 

 foremost portion of the lower jaw has both turned as well as grown upwards and 

 the teeth from one ramus to the other forms an unbroken band. The hook or knob 

 is attached to the whole extent of the groove in front of the jaw and also to a 

 small portion of the superior and inner edge of the lower jaw. Small knobs are 

 occasionally seen at the end of the lower jaw in female trout. 



Respecting fins some authors have seen difierences in the size and shape of 

 the pectorals, as assisting in discriminating a species, but in all forms of oux trout 

 it may be rounded in the very young, become a little more pointed after the second 

 year and again more rounded after the third or fourth season. The outlines 

 shown below are taken from fish between 5-6 and 22 inches in length, from a 

 burn in Stirlingshire, a stream on the Ootteswolds in Gloucestershire, and from 

 two examples of Lochleven trout raised at Howietoun : the features in all being 

 much the same. The number of pectoral rays is immaterial, as I find in my own 

 collection brook trout, and even in the British Museum specimens labelled as 

 8. nigripinnis, 8. ferox, and 8. fario possessuig from 13 to 15. 



Pig. 42. Outlines of pectoral fins of trout. 1, from burn trout 

 from Stirlingshire, male, 5'6 in. long: 2, ditto, female, 6'7 in. 

 long : 3, brook trout from Gloucestershire, male, 8 in. long : 4, 

 ditto, 9-6 in. long ; 5, Lochleven trout from Howietoun, male, 

 12 in. long : 6, ditto, female, 22 in. long. 



* Mr. Arthur, of Otago, wrote on February 10th, 1885, respecting " the hook on lower jaw of 

 males I have never missed examining the heads of all trout that I have seen here, and I never 

 yet saw a male, old or young, which had lost the hook. You may lay it down as a fact that no 

 such thing has occurred here. But I have observed the' greater length of this mark and softness 

 of the point during spawning than during summer among trout of equal weights. I saw a male 

 lately of 10 lb. or 12 lb. where the hooked mandible projected a good half inch beyond the inter- 

 maxillary and yet no indication of anything like shedding it as teeth may be cast." 



