l92 Transactions. — Zoology. 



that no matter how silvery the common trout may have become, its dorsal 

 fin has always a good number of round black spots arranged in rows ; the 

 adipose is brown with more or less of pink round the margin, it also 

 has nearly always two or three dark spots ; the pectoral, ventral, and anal 

 fins are always brown or olive-yellow, occasionally the ventral and anal 

 have a white margin along the longest ray. The maxillary is coarser and 

 generally longer, the suboperculum is somewhat four-sided, with the pos- 

 terior angle more decided than in S. trutta in which it is circular, but this 

 character varies a good deal. The teeth are coarser, and those of the vomer 

 much more permanent than in the sea-trout. I have frequently in old and 

 large fish found them not only on the head but well back along the shaft of 

 that bone. I have also found the fins coarser and larger relatively to length 

 of body, and covered with thicker skin than in the two preceding species. 

 Scales 14 to 17 in transverse row from adipose fin to lateral line. S.fario 

 like 8. trutta is thicker at origin of tail than S. salar. In doubtful cases I 

 have noticed that the head when kept for a week assumed the normal brown 

 colour of the common trout, while the heads of such as I believed to be sea- 

 trout retained their silvery hue. 



In the three species, S. salar, S. trutta, and S.fario, 1 do not believe any 

 dependence can be placed on the numbers of fin rays, vertebrae , or even 

 pyloric caeca (excepting in the case of 8. levenensis), as a means of distin- 

 guishing the one species from the other. Still these have their place and 

 their value, and when any doubt arises as to the identity of any particular 

 Salmonoid, its removal should be the result of a careful consideration of all 

 the invariable distinguishing marks, together with that of such also as are 

 not invariable or not essential. 



I append a table, the investigation of which will bear out generally my 

 observations on the difi'erence in the size of the fins, as also on the ratio of 

 the least depth of the tail to the whole length of the fish. 



