FRESH-WATER TROUT— DESCRIPTION OF. 



197 



males the eye is comparatively smaller, and as much as 3 or 3 J diameters from the 

 end of the snout and also apart (see fig. 19, p. 147). Teeth — in a double row- 

 along the body of the vomer, placed in pairs in the very young and with a trans- 

 verse row across the hind edge of the head of that bone. But as age creeps on 

 the dentigerous ridge along the vomer, as already alluded to (pages 21, 146 and 



No. 1. No. 2. 



Fig. 44. Gill-oovers of brook trout from New Zealand, 

 i natural size. No. 1, male, 20| in. long. No. 2, female, 

 19'2 in. long. 



147 ante), begins to lose some of its teeth (fig. 45, no. 1), and others commence 

 being pushed out from remaining in pairs (fig. 45, no. la) as they had been up to 

 this period. For when the ridge narrows (fig. 45, no. 2), the teeth are of 

 necessity forced to assume a single and zigzag row (fig. 45, no. 2a), and now 

 many fall out, commencing with those furthest from the head of the bone, until 



la 



Fig. 45. Vomers of brook trout. No. 1 from example from Windrush, 

 twice natural size : la transverse section of bone. No. 2 from example 

 from Loch Assynt, twice natural size : 2o transverse section of bone. 

 No. 3 from male from New Zealand, half natural size. No. 4 from 

 female. New Zealand, half natural size. 



nearly all become lost on the body of the vomer (fig. 45, no. 3), and after a time 

 those on the hind edge of the head of that bone as in a male 32|- inches in length. 

 This process may be carried out more rapidly in some forms (especially anadromous 

 ones) than others, and all these figures are from brook trout at different ages, but 

 the process is the same in every variety of trout and even of salmon.* In an 



* Jenyns, 1835, observed that the common trout had "teeth on the whole length of the vomer." 

 Yarrell, 1836, also remarked of the same fish, as well as of Salmo ferox, teeth " extending along the 

 whole length of the vomer." Famell, 1838, of the common trout " vomerine teeth extending the 

 whole way." Dr. Gilnther, 1866, Salmo fario — "vomerine teeth in a double series, sometimes disposed 

 in a zigzag Une, persistent throughout life." S. ferox, " the head of the vomer small, triangular, 

 broader than long, toothless ; the body of this bone armed with a double or zigzag series 

 of teeth, the teeth being alternately placed, forming nearly a single series behind ; they are 



