110 Part III. — T went [/-sixth Annual Report 



much in lue genus. In poutassou it reaches its maximum, and in 

 minutus and luscus it sometimes vanishes through the meeting of the 

 two fins. A useful standard with which to compare this space is the 

 length of the base of the second dorsal fin. 



The Position of the First Dorsal Fin. — In some species this fin 

 begins nearer the snout than in others. In seglefinus, luscus, minutus, 

 and esmarki, the first dorsal fin is placed farther forward than in the 

 other members of the genus, while in poutassou it occupies the furthest 

 posterior position. 



It is an advantage to have the position fixed by comparison with other 

 points on the fish. Thus the beginning of the pectoral fin may be 

 associated with the first dorsal, and the extent of the distance between 

 made a distinguishing character. In some species this character seems 

 to be fairly well fixed, e.g., in aeglefinus, while in others, e.g., esmarki and 

 argenteus, it is subject to a considerable range of variation. In several 

 instances, moreover, it has been found that this distance is relatively 

 greater in large fish than in the small specimens of the same species. 

 This distance has been compared to the length of the snout. 



Latbral Line. — The lateral line is of specific value, both in its shape 

 and in the form of its scutes. The arctic species saida has a very 

 characteristic lateral line ; in it the line forms a bend below the lateral 

 axis, as well as the usual bend above. The line formed by the 

 meeting of the ventral and dorsal muscle segments of the trunk is taken 

 as the lateral axis in this connection. In several species the scutes are 

 noticeable from their wide separation. 



Number of Fin-rays — In some cases it would be possible to fix the 

 species by the number of the fin-rays, but there are cases where the fin- 

 ray formulae of a certain fish might fit into two species. Nevertheless 

 the number of fin-rays forms a very important character. 



Internal Characters. — These do not call for special discussion here. 

 They are included in the key. 



Discrimination-Characters. 



The characters which I have selected, upon which to found a key for 

 the discrimination of the species, are the following: — 



The more anteriorly projected jaw, upper or Lower. 



The position of the anus with reference to the dorsal fins. 



The form of the lateral line. 



The shape of the tail fin. 



The diameter of the orbit compared with the length of the snout. 



The presence and size of the barbel. 



Comparison in length between the pectoral and ventral fins. 



Comparison in length between the pectoral and first dorsal fins. 



The position of the first dorsal fin with reference to the pectoral 



fin. 

 The relation of the second and third dorsal fins to one another. 

 The relation of the anal fins to one another. 

 The relation of the girths at the pectoral and anal regions. 

 Prominent colour marks. 

 Separation of the species by length. 

 The deciduous character of the scales. 

 The number of rays in the unpaired fins. 

 The number of vertebra*. 



The number of the vertebra bearing the first haemal arch. 

 The colour of the inside of the mouth. 



